Microgalvanic corrosion of laser-welded HSLA steels

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Microgalvanic Corrosion

of

Laser-Welded HSLA Steels

door

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and a processed difference image ∆250 obtained by subtracting from an image at 8s from the image at 250s.

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Microgalvanic Corrosion

of

Laser-Welded HSLA Steels

Proefschrift

Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus, prof.dr. ir. J. T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 8 januari 2008 om 15:00 uur

door

Yoke-Moi Looi

Master of Science

University of Manchester Institute of Science and Technology (UK) Geboren te Taiping, Maleisië

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Samenstelling promotiecommissie:

Rector Magnificus Voorzitter

Prof. dr. J.H. de Wit Technische Universiteit Delft, Promotor

Prof. dr. H.S. Isaacs Brookhaven National Lab./Stony Brook University Prof. dr I. Richardson Technische Universiteit Delft

Prof. dr. ir. H. Terryn Vrije Universiteit Brussels, Belgium Prof. dr. R. Boom Technische Universiteit Delft Dr. J. Flores Corus Research Ijmuiden Dr. ir. J.M.C. Mol Technische Universiteit Delft

This research work was carried out under the project number ME 97041 in the framework of the Strategic Research Program of the Netherlands Institute for Metals Research (NIMR) in the Netherlands.

ISBN 978-90-77172-36-0

Keywords: Micro Galvanic Corrosion, HSLA steels, Nd-YAG Laser-Welding

Printed by: Grafisch bedrijf Ponsen & Looijen B.V. Wageningen, The Netherlands

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system from the

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Content - i -

Content

Chapter 1 Introduction 1.1 Introduction 1.2 Research Aim 1.3 Research Approach 1 2 3 3 Chapter 2 Background 2.1 Introduction

2.2 Steels Substrate for Automotive Applications 2.3 Laser Welding

2.4 Corrosion of Polymer Coated Substrates for Automotive Applications 2.5 Vehicle Corrosion Preventions

2.6 Methods of Vehicle Corrosion Testing 2.7 Experimental Techniques 2.8 Conclusions 7 8 9 13 17 25 26 27 32 Chapter 3 Intergranular Corrosion on Nd:YAG Laser-Welded A653 Steel for Automotive Application

3.1 Introduction 3.2 Experimental

3.3 Results and Discussion 3.4 Conclusions 35 36 37 38 45 Chapter 4: Microstructure Characterisation of Welded Joints and Its Correlation with Corrosion Behaviour

4.3 Results and Discussion 4.4 Conclusions 47 48 50 54 64 Chapter 5: The Identification of Anodic and Cathodic Activity Using the Scanning Vibrating Electrode Technique (SVET)

67

68 70

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5.3 Results and Discussion 5.4 Conclusions

75 86 Chapter 6: Corrosion Behaviour of H340LAD in Sodium Chloride Solutions

6.3 Results and Discussion 6.4 Conclusions 89 90 91 94 105 Chapter 7: Initiation and Propagation of Corrosion of Laser-Welded H340LAD Samples

7.3 Results and Discussion 7.4 Conclusions 109 110 110 112 120 Chapter 8: Conclusions 123 Summary 131 Samenvatting 135 Acknowledgements 139 Curriculum Vitae 141

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Chapter 1 Introduction

Synopsis

This chapter provides an overview of the thesis including a general description of the project, research aims and approaches.

Chapter 1: Introduction Chapter 2: Background

Chapter 3: Intergranular Corrosion on Nd:YAG Laser-Welded A653 Steel for Automotive Application Chapter 4: Microstructure Characterisation of

Welded Joints and Its Correlation with Corrosion Behaviour

Chapter 5: The Identification of Anodic and Cathodic Activity Using the Scanning Vibrating Electrode Technique (SVET) Chapter 8: Conclusions Introduction Background Results & Discussion Conclusions

Chapter 6: Corrosion Behaviour of H340LAD in Sodium Chloride Solutions

Chapter 7: Initiation and Propagation of Corrosion of Laser-Welded H340LAD Samples

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1.1 Introduction

Corrosion can be defined as the electrochemical degradation of metals. Automobile manufacturers spent millions of dollars in research on new materials and processing techniques to improve the quality of cars.

Nowadays car bodies are well protected against corrosion by using galvanized steel and by optimised design and coatings. However, corrosion prevention is still important in car manufacturing. Fuel-efficient cars are desired by consumers. For that reason, a lightweight car is needed. Beside lightweight, a car must be crash and impact resistant at an acceptable impact level. In order to achieve the requirement of light-weight and good impact resistance, special high strength steels are applied at areas of car body where good impact resistance is required. In some cases, also thicker materials or specially designed structures will be used while less critical parts can be substituted by applying lighter materials. By joining different materials, weight reduction and at the same time a good safety level can be achieved. For example for car doors, increasing numbers of manufacturers start to use this technique to satisfy consumer needs. However, joining different materials and various thicknesses can provoke new corrosion problems.

Welding is a joining process accompanied by supplying high heat input. The area adjacent to the weld has different properties due to the high heat input and is known as heat-affected zone (HAZ). Literature shows that the HAZs are one of the main areas where corrosion is observed. Thus, techniques that can minimize the HAZs are interesting. So far, laser welding is considered to be one of the preferred alternatives to other welding methods. Laser welding can provide narrower HAZ than conventional welding techniques e.g. tungsten arc welding. The laser welding technique does not only enable joining of materials with different thickness but also allows application of subsequent press forming without further surface finishing. It is not merely an alternative joining technique but a technique that offers fabrication opportunities hitherto difficult or impossible.

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Introduction

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Samples produced by these techniques and used for investigation the behaviour of the welds are known as laser welded tailored blank (LWTB). The neat appearance, strong low distortion weld and efficient production rate, have brought forward wider applications. LWTB can satisfy varying requirements by providing different characteristics to different parts of a single blank. This project focuses on laser-welded galvanized A653 and H340LAD steels which were welded at different welding parameter settings.

1.2 Research Aim

One of the main aims of this project is to study the effect of laser welding parameter settings on the corrosion behaviour of galvanized steels. It is also important to understand how the sample will corrode after the zinc coating is consumed and no longer affords protection to the welded region. Therefore, corrosion mechanisms in and without the presence of the zinc coating have been investigated extensively. A detailed insight in these mechanisms and dominant parameters will further enhance the possibilities for a wide application of these materials and laser welding.

1.3 Research Approach

Figure 1 summarizes the structure of the content of chapters and techniques used for the present investigation.

Chapter 1 provides the general outline of the project and Chapter 2 presents the relevant background information for this project. The initial investigation (Chapter 3) focuses on the as-received laser-welded samples. Comparison of corrosion behaviours on parent metals with and without zinc, and laser welded samples were made based on the results obtained from potentiodynamic polarisation measurements. These experiments help to obtain information on the general corrosion behaviour. Furthermore, corrosion behaviours of samples which were welded by applying different power heat input and welding speed are compared. The surface composition was analyzed by field emission-scanning electron microscope (FE-SEM) and Energy Dispersive X-ray Spectroscopy (EDS).

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After obtaining the general corrosion behaviour of galvanized samples, microstructure characterization on samples was performed and reported in Chapter 4. This chapter also relates the microstructure to the local corrosion behaviour determined using a micro electrochemical cell.

Further investigation of the local corrosion behaviour of as received samples was performed by means of the Scanning Vibrating Electrode Technique (SVET) and results are presented in Chapter 5. This chapter also includes the current density

Chapter 3 Intergranular Corrosion on Nd:YAG Laser- Welded A653

Steel for Automotive Application

Chapter 4 Microstructure Characterisation of Welded Joints and Its correlation with Corrosion

Chapter 5 The Identification of Anodic and Cathodic Activity Using the Scanning Vibrating Electrode Technique (SVET)

Chapter 6

Corrosion Behaviour of H340LAD in Sodium Chloride Solutions

Chapter 7 Initiation and Propagation of Laser-Welded H340LADs Samples

Difference Viewer Imaging Techniques (DVIT)

Scanning Vibrating Electrode Techniques (SVET)

Potentiodynamic Tests Fe-SEM and EDS

Micro Electrochemical Cell Chapter 1 Introduction Chapter 2 Background Chapter 8 Conclusions

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Introduction

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maps of samples, after removal of the zinc coating. The aim of this experiment was to study the corrosion behaviour of laser-welded sample without the influence of zinc coating. Results obtained were correlated to findings in Chapter 4.

Two types of high strength low alloy steels were studied namely A653 and 340LAD steels. Chapter 6 displayed results on A653 and H340LAD samples using the difference viewer imaging technique (DVIT). The zinc coating on samples was removed prior to the experiments. The aim of experiments was to enhance the understanding of the samples without the presence of the zinc coating.

DVIT has also been employed to identify the initiation and propagation of corrosion of laser-welded H340LAD after removal of the zinc coating and results are presented in Chapter 7.

Finally, conclusions have been drawn in Chapter 8. Some suggestions for future work are also included in this chapter.

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Chapter 2 Background

Synopsis

This chapter provides background information on the corrosion behaviour of laser welded galvanized steels for automotive applications. A brief introduction on corrosion of vehicles, corrosion prevention, vehicle corrosion testing techniques, as well as corrosion measurement techniques and materials used for this project is also given.

Chapter 3: Intergranular Corrosion on Nd:YAG Laser-Welded A653 Steel for Automotive Application Chapter 4: Microstructure Characterisation of

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2.1 Introduction

Figure 1: Outline of Chapter 2

Figure 1 shows the outline of this chapter. First, a short introduction of this chapter is described in section 2.1. It is then followed by background information on types of steels used in manufacturing of car bodies (section 2.2). Some background information on laser welding is presented in section 2.3. Sections 2.4 and 2.5 provide background information on the degradation of coated steel substrates for automotive applications and protection against corrosion respectively. Section 2.6 reviews several corrosion evaluation techniques used in the automotive industry while section 2.7 show brief descriptions of experimental techniques used for corrosion investigations for this project. Section 2.8 gives conclusions of this chapter.

2.1 Introduction

2.2 Steels Substrate for Automotive Applications 2.3 Laser Welding

2.4 Corrosion of Polymer Coated Substrates for Automotive Applications

Chapter 2

Background 2.5 Vehicle Corrosion Preventions 2.6 Methods of Vehicle Corrosion Testing 2.7 Experimental Techniques

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Background

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2.2 Steel Substrates for Automotive Applications

The use of lightweight materials has become more prevalent as car manufacturers strive to reduce vehicle weight in order to improve performance, lower fuel and oil consumption and to reduce emissions. This section describes the types of steel substrates used in automotive industries.

Low carbon mild steel has been used for car body panels, with a carbon content of <0.13% to achieve a reasonable combination of yield strength (>140MPa) and formability (n>0.16) [1]. The formability, especially deep drawability of cold rolled low carbon steel strip is maximised by a large ferrite grain size, which is strongly influenced by the concentration and distribution of solute elements especially carbon and nitrogen. Solute elements may also influence the composition, size, morphology, crystallography and distribution of precipitates during annealing [1, 2]. These conditions are controlled by many steel manufacturing and heat treatment processes such as the initial solidification of the steel, segregation patterns, slab reheating, soaking times and temperatures, hot rolling schedules (drafting sequence, finish rolling temperature, post rolling cooling and coiling temperature), cold rolling reduction, annealing parameters (heating rate, annealing temperature and time, cooling rate) for both batch and continuous annealing [1].

For cold rolled and annealed low carbon steel strip it is important to avoid strain ageing1_{, as this causes discontinuous yielding during subsequent deformation in} the forming of car body panels. Strain ageing is caused by interstitial carbon and nitrogen atoms migrating to dislocations resulting in pinning them, thus increasing strength and reducing ductility. This ageing can take place at room temperature and it may take several months for the strain ageing effect to occur. Strain ageing can be avoided by ensuring that there is no free interstitial2_carbon or nitrogen, by reducing the total C and N content to very low levels (0.003%)

1

Strain aging: The gradual changes in physical and mechanical properties, in particular hardness and tensile strength, which takes place following cold rolling or deformation. At atmospheric temperatures, this may take place over a number of weeks but can be accelerated by heating.

2

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through modern secondary steelmaking practices and through the addition of strong carbide and nitride forming elements, such as vanadium, niobium and titanium. Aluminium and boron can also be used to form nitrides. This approach is called stabilisation. Overageing3_{, as part of the annealing process, can also be} used to form carbides and minimize free carbon. This type of steel is known as interstitial-free (IF) cold rolled steel. These steels have a low strength and excellent ductility and formability. A typical IF steel composition is 0.002%C, 0.01%Si, 0.15%Mn, 0.01%P, 0.01%S, 0.0025%N, 0.04%Al, 0.016%Nb, and 0.025% Ti [1]. In this steel category, one common element added to increase its strength is phosphorus (a solid solution strengthener). This steel type is widely used for both structural and closure applications [3].

"High Strength Cold Rolled Steel Strip" has a yield strength >210MPa. This type of steel are commonly applied in automotive and tube making industries. There are various metallurgical options for increasing the strength of low carbon and ultra-low carbon steels. Each produces a different combination of strength, ductility, stretch formability and drawability and with different cost implications. The properties are also strongly influenced by the hot and cold rolling and annealing parameters.

"High Strength Hot Rolled Steel Strip" utilizes precipitation strengthening through the addition of microalloying elements (niobium, vanadium and/ or titanium, each up to 0.1%), which form fine carbides or nitrides. These are called High Strength Low Alloy (HSLA) steels [1, 4, 5]. After cold rolling and annealing the dispersion strengthening effect of these elements is usually lost through particle coarsening, but nevertheless, the resulting fine-grained HSLA steels have attractive combinations of strength and formability. Typical compositions are: 0.05-0.1 %C, 0.25-1.2%Mn, 0.01-0.05%Nb, 0.01-0.4%Si for Nb alloyed HSLA grades. Sample A653 and H340LAD used for this project are HSLA steesl which consist of composition within the range mentioned. Detail of composition can be found in Chapter 4.

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Background

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Several "Advanced High Strength Steels" have been developed. These include Dual Phase (DP) steels that are characterised by a ferrite-martensite microstructure, which exhibits low yield strength but a very high work hardening rate so that the stretch formability is excellent and a high strength is achieved in the formed component. In cold rolled steels the dual phase microstructure is created by continuous annealing in the intercritical temperature range followed by rapid cooling. The alloy content of the steel has to be carefully selected to generate a level of hardenability that will enable the martensite transformation to occur during cooling and coiling. The properties are controlled by the relative proportions of the two phases and their size and morphology. For a similar strength to HSLA steels, Dual Phase steels offer superior formability and enhanced crashworthiness [5]. Typical compositions for cold rolled Dual Phase steels are 0.08-0.18%C, 1.6-2.2%Mn, 0.4%Cr+Mo. A 0.05%C 1%Si, 1.5%Mn, 0.6%Cr and 0.4%Mo steel is typical of a hot rolled Dual Phase Steel.

Another Advanced High Strength Steel is TRIP steel (Transformation Induced Plasticity). These exhibit mixed microstructures of ferrite, bainite, martensite and retained austenite, which transform during forming to martensite, giving a high work hardening rate and high tensile strength [3]. Consequently, they have better formability than Dual Phase steels, particularly at high levels of strain (n values of >0.20) and with similar high strength potential. TRIP steels need higher silicon and /or aluminum and/or phosphorus levels than Dual Phase steels. They are however difficult to produce. Some complex high strength steels manage to combine the microstructural features and high work hardening rate of Dual Phase and TRIP steels with an additional precipitation strengthening. Typical compositions of TRIP steels include "Silicon alloyed TRIP" 700/800: 0.2%C, 1,5%Mn, 1,5%Si or "Aluminum alloyed TRIP" 600: 0.2%C, 1,5%Mn and 2,0% Al. Other types of car body steels are Complex Phase (CP), Bake Hardenable (BH), High Strength Carbon-Manganese and Martensitic (MART) steels. CP steels consist of a very fine microstructure of ferrite and a higher volume fraction of hard phases that are further strengthened by fine precipitates. Complex phase steels typify the transition to steel with very high ultimate tensile strengths. BH

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steels have a basic ferritic microstructure and are strengthened primarily by solid solution strengthening. A unique feature of these steels is the chemistry and processing designed to keep carbon in solution during steelmaking and then allowing this carbon to precipitate from solution during paint baking. This increases the yield strength of the formed part [3]. High strength carbon-manganese steels are primarily strengthened by solid solution strengthening. To create martensitic steels, the austenite that exists during hot rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. This structure can also be developed with post-forming heat treatment. Martensitic steels provide the highest strengths, up to 1700 MPa ultimate tensile strength.

Figure 2 shows strain-hardening and yield strengths of different steels. According to the UltraLight Steel AutoBody (ULSAB) consortium formed for the ULSAB program, any yield strength higher than 550Mpa is called ultra high strength steel [5]. The minimum yield strength for high strength steels is ranging from 210MPa to 420MPa while for mild steel this is about 140MPa.

Due to the possiblity of achieving high strengths at a formed component, ultra high strength steel has gained popularity in the automotive industry for the Figure 2: Chart showing the relationship between strain hardening exponent, n and yield strength for a range of cold rolled strip steels [1].

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Background

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The materials used in this project are hot-dipped galvanized A653 and H340LAD steels. They were both high strength low alloy steels. Details on the composition is presented in Chapter 3 and 4. The galvanized layer thicknesses are approximately 6.3µm and 10.0µm pure zinc respectively. These materials were Nd:YAG laser welded and cut into surface area of 1 x 2 cm2_{. Due to the high heat} input during the welding process, most zinc evaporates and as a consequence, the weld becomes an area exposed to the substrate. Besides, high heat during welding results in presence of heat-affected zones which are situated adjacent to the welded seam. These zones influence the corrosion behaviour of the samples due to alteration of material and mechanical properties i.e. microstructures and stresses. Hence, this project is focused on how welding affects the behaviour of laser welded galvanised steels.

2.3 Laser Welding

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation [6-11]. The laser technology was conceived by Townes in 1951. In May 1960, T. H. Maiman of Hughes Aircraft Research Laboratories in California demonstrated a device, working in the visible region of the spectrum, utilising a synthetic ruby crystal excited by a gas discharged flash tube and emitting short pulse of red coherent light [7-10]. In 1961, Javal of Bell Labs produced a laser beam from a mixture of helium and neon gases excited directly by an electrical discharge. The CO2 laser was later developed by Patel of Bell Labs in 1964, which has since become the industrial workhorse. The latest development, which has increasingly gained industrial interests, is the neodymium glass and the neodymium yttrium aluminium garnet (Nd:YAG) [9]. Laser, since its introduction, has been applied in a wide variety of areas such as communication technology, in surveying, in medical, electronic, and domestic appliances for metal working as well as in automotive industries [8, 9]._{Laser welding is increasingly popular in automotive} industries due to its possibility to produce a single steel sheet of different thickness of materials. In the ULSAB program, Porsche Engineering Service Inc. decided to adopt this technique for several reasons [3]:

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i. Mass reduction due to the possibility of placing optimum steel thicknesses and grades was needed.

ii. Elimination of reinforcements with appropriate material gauge selection.

iii. Simplified logistics due to the reduction of parts steps (Laser welding can produce a single sheet with different thicknesses of materials, which can then be pressed to form the required parts. Using conventional methods, transfering parts from one fabrication area to another may be required. Since this method reduces the steps required, a logistic cost saving is evident).

iv. Better corrosion protection by the elimination of overlapped joints. v. Improved structural rigidity due to the smoother energy flow within the

tailor welded blank parts.

vi. Better fatigue and crash behaviour compared to a conventional overlapped spot welded.

vii. Reduction of investment costs for dies, presses lower production design solution because there is also a possibility of press forming on a laser welded tailor-blank sheet without resulting in welded joint distortion. As such, costs for conventional production methods such as casting, machining and assembly in individual sections can be greatly reduced.

2.3.1 Principle of laser welding

Laser welding is achieved by creating a liquid pool of steel by absorption of incident radiation, allowing it to grow to a desired size and then propagating this pool through the solid interface eliminating the original seam between the components [6]. There are two fundamental modes of laser welding namely; conduction welding and keyhole welding (Figure 3). The first mode offers less perturbation to the system because laser radiation does not penetrate into the material being welded. As a result, conduction welding welds are less susceptible to gas entrapment during welding. The keyhole welding mode provides narrower heat-affected zone (HAZ) than the conduction welding mode. Due to this reason, research has focused on the improvement of this technique. In order to form a

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Background

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keyhole mode laser weld, the laser beam is brought to focus on or very close to the surface of workpieces to be joined. In the first instance, a large percentage of the incident beam is reflected from the surface for a minute period, because most metals are good reflectors [9]. However, the small amount of laser beam energy which is initially absorbed by the work quickly heats the material surface, causing production of an energy absorbing ionized metal vapour, which rapidly accelerates the absorption of much of the energy that previously would have been reflected. As the focused power density is in the order of 104_W/mm2_{, the} rapid removal of metal by vaporization initiates a small keyhole (a very small diameter cylindrical shaft) into the work piece. As the keyhole penetrates deeper in to the workpiece, the laser light is scattered repeatedly within it, thus, increasing the coupling of laser energy into the weld. As a rule of thumb, 1kW of correctly focused laser light will comfortably weld to a depth of 1.5mm at a speed of 1m/min [9].

2.3.2 Types of Joints for Welding

A number of standard joint geometries can be laser welded. The most common are butt, T-butt, and lap (Figure 4) joints. A bead-on-plate joint is mainly used for testing. Butt and T-butt joints are produced from welding two pieces of materials together, whereas two or more materials can be welded by lap joints. Bead-on-plate joints are produced on a single piece of material. A butt-joint, however, is the geometry encountered in blanking operations (blanking operations are processes when the tailor-welded blank is press forming), particularly with laser welded tailored blanks (LWTB) [6]. Full penetration must be maintained to achieve good mechanical properties from butt-welded seams. Clamping to maintain the fit-up (the arrangement of metals to be welded. See ‘Butt Joint’ in Figure 4) is of utmost importance in butt-welding. The sheets have

(a) (b)

Laser Beam

Weld pool HAZ

Keyhole

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a tendency to misalign during welding due to thermal stresses and distortion; for this reason, long-seam butt welds in thin sheet material are often tack welded before welding the seam.

Figure 4: Common joints in laser welding process

2.3.3 Laser Welding of Galvanised Steels

Surface coatings are widely used on steel to provide corrosion resistance and a good surface finishing. Laser welding of thick steel (0.8-1.5mm) metals parts coated with zinc is becoming more common especially in the automotive industries [9]. It is reported that successful application is dependent on the coating thickness, its uniformity and especially joint configuration used [9]. Butt joining, where the zinc coating does not exist at the joint interface, normally presents no serious problems to laser welding. Lap joints between single side zinc coated steel sheets where the coatings are on the outside face, weld similarly to butt weld. However, when the zinc coating is at the joint interface, serious problems which affect weld formation arise. This problem is because zinc boils at 900˚C and steel melts at 1500˚C. Consequently, zinc at the interface will vaporise and pressurise before the weld keyhole is formed. Once the keyhole is formed, bubbles of vaporised zinc will evolve. Any trapped bubbles will usually be very small and well dispersed along the length of the weld [9]. Laser welding can minimise the size of the heat-affected zone and thereby reduce the materials distorted area. On the other hand, when applying laser welding on galvanized steel, a weld seam with almost no zinc due to vaporisation may be produced. This condition in the presence of an electrolyte leads to a potential galvanic cell.

GAP GAP GAP

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Background

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2.4 Corrosion of Polymer Coated Substrates for Automotive Applications

This section gives a general background of polymer coated steel substrates and background information on car corrosion. It is then followed by description the types of car body corrosion and its relationship to the degradation of coating systems.

2.4.1 Polymer coated steel substrate

As mentioned in the previous section, a zinc coating is applied to the car body steel sheet to provide improve corrosion resistance. A complementary corrosion barrier for a car body is a good polymer coating layer. This coating layer consists of a primer and top coat. Phosphating is performed on the galvanic layer to improve the adhesion between the polymer coating and the metal substrate. Phosphating is carried out in mild alkaline phosphate baths followed by the final phosphating treatment in a mix-spray-dip process. This process leads to formation of a phosphate crystal layer which can be either hopeite [Zn(PO4).4H2O] or phosphophyllite [Zn2Fe(PO4)2.4H2O] depending on the composition of the substrate [12].

Excess phosphoric acid is washed off with water and the vehicle body is carried by an overhead conveyor for application of subsequent electro-coating processes [13]. The vehicle body is then rinsed several times, both with water and with a weak paint removal solution to remove a thin coat of paint that clings onto the surface but has not bonded to the metal. If this is not removed, then the car body would have an uneven, messy coat of paint. The car body is then baked to 170 - 180˚C in an oven [12, 13]. This causes the different polymer chains to crosslink forming a very strong, flexible, interconnected network of polymer over the whole surface of the car. Figure 5 shows a schematic diagram of a typical coating system [12]. Often two layers of top coats are applied; a base and clear coat.

Figure 5: A schematic cross section view of a coated automotive material [12]

Note: Fillers: Particles added to a matrix material, usually to improve its properties. Examples of fillers are carbon black, natural mineral fillers and synthetic mineral fillers.

Steel (Cold Rolled Steel) Electrocoat (20µm) Fillers (40µm)

Top Coat (40µm)

Zinc coating (10-12µm) Phosphate layer

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Corrosion prevention in the automotive industry is an important technical challenge for extended vehicle life, safety, weight reduction and cosmetic appearance [14, 15]. Vehicles are often exposed to a variety of harsh atmospheric conditions. Typically, the combination of deicing salts, sorption of corrosive gases, and moisture provides the appropriate condition for severe corrosion [14]. Figure 6 shows some examples of corrosion at the body of vehicles. During the late 50s in the United States, corrosion of vehicles was a concern limited only to application in a marine environment. However, with the increasing use of deicing salts, vehicles were found to corrode more severely [15]. In the late 70s, automotive manufacturers started activities to improve the corrosion resistance of car bodies by improved design and manufacturing processes, the use of modern materials such as high quality steel, applying modern surface technology, better welding techniques and corrosion protective coatings [12, 14-28]. Due to these efforts, today’s vehicles have a tremendously improved corrosion resistance.

2.4.2 Types of Vehicle Corrosion

The types of corrosion of automotive bodies can be classified broadly into two categories; cosmetic corrosion and perforation corrosion [29-31]. Cosmetic corrosion occurs when stones, gravel or other objects strike the outer panels of the vehicle, damaging the paint film and allowing the formation of red rust or scab-like rust (blisters). Perforation corrosion occurs at body parts designed such that dirt, water and salts easily accumulate e.g. in flanges (Figure 7). These parts experience a different corrosion type compared to that suffered by a chipped car body. Examples are doors, hoods, chassis and frames: the resulting

Figure 6: Vehicle Corrosion left to right: Corrosion on a bus body, and corrosion at reflector.

Corrosion

Reflector Car

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Background

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corrosion initiates on the inside and proceeds to reach the outer surface of the material [30]. Figure 7 shows an example of a flange and a potential location for dirt and liquid accumulation. This kind of corrosion can propagate virtually undetected until the panel actually perforates [4, 12, 14, 15, 28]. Both cosmetic and perforation corrosion can lead to the disbondment of a coating system.

2.4.3 Mechanism of Coating Disbondment

Granata described nine mechanisms of coating disbondment [33]:

i. Anodic undermining – results from the dissolution of the metal beneath the phosphate layer and is associated with the presence of cathodic regions formed at defects where oxygen concentration level is high or the metal coating becomes galvanically coupled to the more noble steel substrate in automotive system. This process is associated with acidification resulting from hydrolysis of metal ions formed in the anodic reaction.

ii. Thermal cycling - alone or in conjunction with other disbondment processes, causes stresses on the coating which can break adhesive bonds to the substrate.

Figure 7 [32]: Example of a flange. A flange consists of an inner and external panel. A poorly designed flange can result in accumulation of water, salts, mud at gap or cavity.

Potential cavity for water, salt and mud accumulation

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iii.

Cathodic blistering (see also cathodic delamination) - is the formation of blisters related to the electrochemical reduction of oxygen thereby locally at the edge of th eblister increasing the pit beneath a coating initially having no physical damage. Thus delamination at the edges of the coating is stimulated by the high pH attacking the polymer. Oxygen, water and some ions diffuse through the paint while the centre of the blister is considered to be the anode due to passivation as a result of the initial formation of lower values Fe oxided, that can consume oxygen. The phosphate layer would also suffer damage and would provide minimal protection [12, 33, 35]. Pressure built up by voluminous corrosion product formation at the coating/metal interface will lead to further deterioration of the adhesion of the coating system and eventually leads to failure. This process is nearly always

Blister

Figure 8 [34]: Schematic of cathodic blistering mechanisms of coating system failure of an intact coating, with initiation site at the polymer/substrate interface where the chemical bonding Is not ideal. 8(a) Ingression of reactants into the coating. 8(b) Blistering of coating due to electrochemical process and pressure at the coating/metal interface. 8(c) Failure of the coating leads to corrosion processes at the galvanic layer and eventually also at the surface of the base metal.

(a)

(b)

(c)

Infusion of H2O, O2, CO2, H2S, Na+, Cl

-Intact coating on metal

Burst of blister Infusion of H2O, O2,

CO2, H2S, Na+, Cl

-High pH developes at these points due to either O2+H2O+ 4e-= 4OH-or 2H++2e-=H2 Steel

Galvanic coating Phosphate layer Electrocoat Filler or primer surfacer

Top Coat

Steel Galvanic coating Phosphate layer Electrocoat Filler or primer surfacer

Top Coat

Steel Galvanic coating Phosphate layer Electrocoat Filler or primer surfacer

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Background

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accompanied by osmotic blistering (see the description of osmotic blistering).

iv. Swelling of the polymer in some paint films - is due to the uptake of water and induces stress in the coating leading to possible disbondment in coating systems with poor adhesion.

v. Gas-blistering - is unusual but technically possible especially with pure Zn coatings and due to hydrogen ion reduction to form H2. It is related to acidic environments where hydrogen ion reduction is more likely than oxygen reduction [33, 35].

vi. Osmotic blistering - is a process by which blistering occurs due to high water pressure induced by soluble materials beneath polymer films. For instance, the presence of soluble salt at the coating/substrate interfaces [35]. As water penetrates the coating to the interface, a concentrated solution is developed with sufficient osmotic force to drive water from the coating surface to the interface and a blister is formed. vii. Cathodic delamination (Figure 9) - is the mechanism by which a

cathodic process causes disbondment leading to the separation of a coating from a metallic substrate [33-35].

The main feature of the mechanism is the reduction of oxygen at the coating/substrate interface:

O2 + 2H2O + 4e- =4OH- Infusion of H2O, O2, CO2, H2S, Na+, Cl

-High pH developes at these points due to either O2+H2O+ 4e-= 4OH-or 2H++2e-=H2 Steel

Defective Coating Galvanic coating

Phosphate layer Filler or primer surfacer

Top Coat

Electrocoat

Figure 9 [33, 34]: Schematic cross-section view of coated steel substrate damaged by impact resulting in cathodic delamination.

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Oxygen and water are capable of diffusion through polymer coatings. The electrons are provided by the metal substrate conductivity from the anodic sites. The degree to which the electrochemical reaction causes damage to the coating/metal interface depends upon the nature of cations present at or transported to the interface. The greatest damage occurs with alkali metal cations such as sodium and potassium. High alkalinity is greatly damaging to many polymers; dissolves oxides, phosphates and many types of surface treatment layers (Figure 5). This condition promotes the corrosion of Fe and Zn by shifting the electrode potential to more active values. If there are no defects in the film, the reactants slowly diffuse through the coating (see the description of cathodic blistering). However, in the presence of a defect, the reactants rapidly reach the interface through the defect. In the event of perforation, the results are cathodic blistering formation with little or no rust until the blister breaks [12, 33-35].

viii. Mechanical delamination - is the disbondment of the paint film from the substrate caused by corrosion product formation. The significance of this type of delamination is the relationship to corrosion processes generating stresses resulting in the propagation of coating system damage (see the description of cathodic blistering).

ix. Combined effects – cathodic delamination at the paint/phosphate-metal interface, anodic undermining at the delamination front (loci of chemical attack) and mechanical delamination associated with voluminous corrosion product precipitation from the area of original paint film damage provide a catalytic effect on the overall disbondment mechanism under cyclic environmental conditions. The mechanism has a catalytic component in the sense that upon completion of each wet/dry cycle, the process proceeds at an increased rate due to reactant re-supply at the delamination front.

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Background

- 23 -

2.4.4 Galvanic Corrosion

A galvanic cell is created when dissimilar metallic materials are brought into contact with each other in the presence of an electrolyte [32, 36, 37]. The more active metal of the two in the galvanic series becomes a sacrificial anode and cathodically protects the other. The surface area ratio of the two dissimilar materials is extremely important. If the anode-to-cathode surface area ratio is small, the galvanic current will concentrate on a small area, resulting in an extreme rate of materials loss which makes ‘cathodic protection’ shortly ineffective and may even harm a construction. For example, if aluminium rivets were used on steel plate, the rivets would corrode rapidly resulting in a collapsing structure. Galvanic corrosion effects can become manifest not only on a macroscopic level but also within the microstructure of a material (Figure 10). Certain phases or precipitates will undergo anodic dissolution under microgalvanic effects. For example formation of chromium carbide along the grain boundaries at the HAZ of austenitic stainless steels can lead to microgalvanic corrosion. It is reported by Rothwell et al. that galvanic cell can also be generated by impurities present on metal surface or molten metal made up of metal from filler metals during welding [38]. Another example is resulting from the diffusion of fresh, fused material at the edge of the weld pool where a narrow band formed (also known as fusion line) with a composition gradient. Under some conditions, these regions may be strongly anodic to both parent metal and the weld material and consequently galvanic corrosion proceeds. Such corrosion attack is also known as fusion line attack.

Zinc Steel

(a) (b)

Figure 10: Types of galvanic corrosions. (a) shows example of macrogalvanic cell while (b) shows a microgalvanic cell occurs due to the precipitation of chromium carbide at the grain boundaries of stainless steel after welding.

Weld HAZ

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Understanding of the microgalvanic effect in welded joints is vital as laser- welded samples were used for the investigation. Figure 11 represents a schematic polarisation plot for a parent metal and a HAZ at which HAZ covers only a small area compared to the parent metal and they are corroding separately. Assuming the HAZ is anodic to the parent metal; the corrosion potential (E HAZ) is more negative than that of the parent metal (E parent metal). Even though the corrosion current of HAZ is smaller than that of the parent metal, the current density of HAZ should be larger due to the area difference. In practice, parent metal and HAZ co-exist and this couple would corrode at E couple which is higher than the corrosion potential of HAZ but lower than that of parent metal, according to the mixed potential theory. The anodic current of parent metal, is reduced from I parent metal to I parent metal couple. The anodic current of HAZ is increased from I HAZ to I HAZ couple. The relative intensity of attack depends on their relative areas. Since the HAZ area is smaller than the parent metal, the HAZ would corrode preferentially at a rate many times higher than the corrosion of the parent metal.

Eparent metal + E -EHAZ Ecouple I Parent metal I HAZ I couple Total oxidation Oxidation parent metal

Total reduction Reduction HAZ

Reduction parent metal

Oxidation HAZ Parent metal couple I HAZ couple I I

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Background

- 25 -

2.5 Vehicle Corrosion Preventions

Recent reports on the cost of corrosion of motor vehicle have discussed the major changes in automobile corrosion prevention since 1975, predominantly being (i) the use of materials, (ii) better surface finishing processing techniques and (iii) improved design [39].

i. Materials:

The changes in materials usage are summarised in Table 1. This table shows the average weight of each material and the weight percentage of each material for a typical family car in 1978, 1985 and 1996. The percentage shows that percentages of regular steel and iron have reduced from 67.9% in 1978 to 55.5% in 1996. The use of high strength steel, stainless steel, plastic, aluminium, and copper, on the other hand has significantly increased from 1978 to 1996. These materials have replaced mild steels for greater strength, weight reduction and corrosion resistance.

ii. Processing Techniques

Application of zinc phosphating has been introduced to improve the paint adhesion [39, 40]. Prior to 1975, all body paints were applied with an

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spray atomizer which gave a good finish on the exterior of the car body but the interior area often received no coverage, which led to corrosion prone areas. In 1976, cathodic deposition was introduced to ensure an overall and even coverage. Sealing of car body and exposed flanges to reduce cosmetic and perforation corrosion was manually performed but has since become a robotic operation to ensure quality of the sealing job. Augmentation coatings such as anti-chip plasticsols4_{and urethane coatings are applied at the rear of} the wheel house before final painting. Application of waxes to the interior cavities using automated wax coverage is also applied to reduce operator error. The final surface finishing process is the application of the top coat. The top coat is applied using robotics and control equipment resulting in a more uniform paint coverage.

iii. The improved vehicle design e.g. minimisation of crevices and bimetallic contacts as well as optimisation of the paint process have resulted in increased finish quality and corrosion resistance [39, 41].

2.6 Methods of Vehicle Corrosion Testing

Corrosion of vehicle tests can be categorized into vehicle and part/material tests (Figure 12). The vehicle tests include field monitoring and proving ground tests [14]. Field monitoring tests provide accurate evaluation as they are performed in severe environment, especially at the area using high amounts of deicing salts.

4_{Modified PVC or PMA dispersion which requires heat to harden. The resultant joints are often}

Vehicle

Part / Material Corrosion

Tests

Proving Ground Test Field Monitoring Test

Laboratory Test

Outdoor Exposure Test

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Background

- 27 -

However, this type of test is time consuming and costly. Proving ground tests are implemented in each vehicle model. This type of tests includes road tests on test tracks which reproduce actual road conditions, such as salty-muddy and gravel road, as well as corrosion accelerating tests by cyclic whole vehicle exposure to a salty, high temperature, high humidity environment.

Accelerated tests for materials or parts are critical for the efficient development of new corrosion resistance materials. The salt spray test (SST) is one of the most popular methods for this type of testing. However, this method does not provide a good correlation with the type of corrosion that occurs in the field. Cyclic Corrosion Tests (CCT) better resembling practical conditions, have therefore been implemented and are more widely used [23]. Outdoor exposure tests can provide good data but are also very time consuming.

For this project, none of the industrial tests mentioned in this section are utilised as the main aim of the project is to identify the corrosion mechanism at a welded joint. More discriminative measurement methods are needed which including Scanning Vibrating Electrode Technique (SVET), Micro Electrode Cell (MEC) and Difference Viewer Imaging Technique (DVIT). The background of these experimental techniques to elucidate the corrosion mechanisms at welded joints are presented in section 2.7.

2.7 Experimental Techniques

In this section, an overview of techniques used for the investigations are described. The first stage of this work was carried out using macro electrochemical techniques which provides an overall electrochemical behaviour of the samples. Subsequently, the Scanning Vibrating Electrode Technique (SVET), and Micro Electrode Cell (MEC) were employed to observe the corrosion behaviour in the micrometer range. Difference Viewer Imaging Techniques (DVIT) was used to identify early corrosion initiation and propagation. Detail descriptions of techniques can be found is the following chapters.

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2.7.1 Macro Electrochemical Techniques

Direct current (DC) electrochemistry, in particular potentiodynamic polarisation, generates considerable information on the overall electrode processes. Through the DC polarisation technique, information on the corrosion rate, pitting susceptibility, passivity as well as the cathodic behaviours of an electrochemical system may be obtained. In order to induce an electrochemical process to take place, there must be an anode, a cathode as well as both an ionic and electronic conduction path between the two. When performing a DC polarisation scan, the ionic conduction path is provided through the electrolyte separating the working and counter electrodes, while the electronic conduction path is provided through the potentiostat. The potentiostat is then used to control the electrode potential relative to a reference electrode for the electrochemical reactions taking place at the working electrode. The magnitude of this driving force in turn dictates the electrochemical processes actually taking place at the working electrode surfaces. Results are recorded by a computer or laptop.

Both potentiodynamic (ramped potential) and potentiostatic polarisation (fixed potential) were used to study the macroscopic corrosion behaviour. The potentiodynamic test enables observation of the general corrosion behaviour of the material. Based on the results obtained from potentiodynamic tests, potential ranges, which show interesting features such as breakdown potential, can be identified. Potentiostatic tests performed, based on these observations, are used to actively degrade the test material, producing the degradation products to be analysed. These electrochemical techniques were accompanied by optical microscopy and field-emission scanning electron microscopy.

Both types of measurements were generally conducted after stabilisation of the sample surface conditions which at the open circuit potential (OCP). The electrolyte can be stirred to reduce concentration polarisation. Close reference electrode/working electrode spacing was used to reduce the IR (ohmic) resistive voltage drop across the cell.

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Background

- 29 -

2.7.2 Micro Electrode Cell

The Micro Electrode Cell (MEC) is a technique, which was developed to perform standard electrochemical measurements including OCP, potentiodynamic, and potentiostatic polarisation [42]. However, unlike the other techniques, this technique can be used to measure areas with dimensions in the micrometer range. The miniaturisation of the exposed area of the working electrode is obtained by a micro glass pipette to position an electrolyte contact on the sample surface. This droplet of electrolyte is held in position by a silicon rubber ring which is located at the mouth of the micro pipette. The internal diameter of the mouth of the micro electrode can be as small as a few microns which make this technique an interesting tool to study local electrochemical behaviours of intermetallics [43-45]. Due to the interesting feature of this technique, it was used in this project to study the corrosion susceptibility of microstructures of welded samples. Further details of this technique canbe found in Chapter 4.

2.7.3 Scanning Vibrating Electrode Technique (SVET)

SVET is an electrochemical method that was used to investigate the corrosion phenomena and provided information on the current and potential of a corroding surface or an electrode in an electrochemical cell [46]. The scanning vibrating electrode technique enables by scanning over the surface, the measurement of local current density with a typical lateral resolution from 1000 µm to 10µm depending on the conductivity of electrolyte and the size of the scanning tip.

vibrating micro-electrode probe

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This technique employs a micro-electrode that vibrates from position A and B (Figure 15) to measure the potential gradient (electric field) in the solution above the sample. The vibration of the micro probe converts the IR drop into the AC voltage with amplitude equal to the DC potential difference (∆V) over the distance of vibration (d in Figure 15) [47]. The AC voltage has the same frequency of the micro probe vibration. A piezo-crystal is used to generate vibration for the micro probe at both perpendicular and parallel directions to the sample surface. Current density vectors can then be calculated from the DC potential difference at every position by vibrating the probe in two different directions. An upward vector represents an anodic current (positive current) while a downward vector represents a cathodic current (negative current). A lock-in amplifier is used to amplify the signal at the frequency of vibration and to filter out other frequencies, making the equipment a sensitive tool. Figure 16 shows an example of the plotting vectors. A detailed description of this technique is presented in Chapter 5.

2.7.4 Difference Viewer Imaging Technique (DVIT)

The DVIT technique is implemented based on the principle of pixel values of Working

Electrode

Figure 16: Plotting of vectors on a corroding welded sample superimposed on a micrograph of the exposed sample. A higher anodic current is found at the position of the weld. Weld Electrode/ Probe Sample More positive Anodic current Less anodic Current current

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Background

- 31 -

and Blue (RGB) values. An orange colour as shown in Figure 18 is made up of RGB values of 247, 103 and 15 respectively. White, grey and black have RGB value of (255, 255, 255), (128, 128, 128) and (0, 0, 0) respectively. All colours with RGB values above (255, 255, 255) will be white and colours below RGB values of (0, 0, 0) will be black/dark. When corrosion occurs, some corrosion activity can be observed like generation of bubbles on the corroding surface and very often the surface sample turns dark due to formation of corrosion products. Such darkened surface is at the limit of human eye sight to further observe surface changes.

DVIT is a fast and non destructive in situ visualisation technique to study the corrosion of materials [48]. Image acquisition and processing are controlled by a programmed computer. Colour images are continuously collected using a digital camera mounted on a microscope during electrochemical measurements. Two live displays are presented on the monitor, the real image and a difference image. The difference images are obtained by digital subtraction of the real-time image from a previously acquired image, amplification of the difference, and offsetting of the result against a grey background. The difference image shows only the changes that occurrs in the period between the recording of the previous image and the real-time image. One major advantage of the approach is that very small changes can be located and monitored over very short periods of time e.g. 4 seconds. Without subtraction, the monitoring of change is

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extremely difficult, as the details of the image itself may create a “noisy’ background that camouflages the changes taking place. Due to these features, DVIT is an interesting technique to study local corrosion initiation and propagation. More details of this technique are given in Chapter 6.

2.8 Conclusions

Protection of car bodies against corrosion has been improved through better design and the use of corrosion prevention measures including proper coating technuqes; ultra high strength steel is gaining popularity in automotive industries to manufacture light weight and fuel efficient cars. However, new or improved processing methods such as welding may again provoke different forms of corrosion; welding changes the properties of materials as a result of heat input. Knowledge of corosion degradation mechanisms as described can enhance the understanding of potential corrosion attack which might occur in a laser-welded galvanised steel joint. Novel electrochemical techniques have been developed in the past decades which enable detailed studies on the corrosion processes resulting from the new materials processing works. These techniques will be further described in the remaining chapters.

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Background

- 33 -

References

1. W.C.Jeong, Metallurgical and Materials Transactions A, February, 464-467, (1998) 2. W.F. Smith, Principle of Materials Science Engineering, McGraw-Hill Inc, (1996). 3. International Iron and Steel Institute, Advanced High Strength Steel Application

Guideline, 18 March 2005.

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(1989).

9. C. Dawes, Laser Welding a Practical Guide, Woodhead Publishing, Cambridge (1992).

10. E. Linnert, Welding Metallurgy Carbon and Alloy Steels, fourth edition, American welding Society, Florida (1994).

11. M. Naeem; R. Jessett, Practical Welding Today (USA), 3, 24-28 (1999). 12. A. Amirudin, D. Thierry, Prog. In Org. Coatings, 28, 59-76 (1996).

13. New Zealand Institue of Chemistry, Chemical Processes in new Zealand Second Edition, Chapter 10 ‘Polymer and Surface Coating’, 1998

14. K. Okazaki, Recent Advances in Coated Steels Used for Automobile, 49-68 (1996). 15. J.T. Johnson, http://www.corrosioncost.com/transportation/motorvehicles/, 2005. 16. R. S. Michalak, American Society for Metals, 115-131 (1981).

17. R. Baboian, Materials Performance, 34, 48-52 (1995).

18. M. Uchida; K. Mochizuki, Kawasaki Steel Technical Report (Japan), 43, 14-20 (2000).

19. R. J. Neville, SAE No: 800144, 35pages (1980). 20. R. J. Neville, Metal Progressing, 107, 71-73 (1975).

21. L. Allegra, H. E. Townsend, Conference: Conference on Efficient Materials and Coatings Applications for Improved Design and Corrosion Resistance, Chicago 13-15 Nov 1979, 13-157-164 (1981).

22. K. Partington, Anti-Corrosion methods and Materials, 15, 17-19 (1968).

23. F. Blekkenhorst, E.Nagel Soepenberg, M. Roelofsen, J. P. Schoen, Nace Annual Corrosion St. Louis Missouri 1988, Paper No. 354 (1988).

24. S. Hedlund, Report on Rust Status of Cars with anti-rust Guarantee in the Nordic Countries (1993).

25. R. Winston Revie, Uhlig’s Corrosion Handbook, 2nd_{edition, John Wiley & Sons Inc.,}

New York (2000).

26. L. C. Rowe, Automotive Engineering, 82, 40-45 (1997).

27. Automotive Steel Design Manual, American Iron and Steel Institute & Auto/Steel Partnership, Revision 5 (May 1996).

28. L. A. Roudabush, T. E. Dorsett, Proc. 5th_{Automotive Corrosion Prevention Conf.}

SAE Dearborn, Paper No.912285, 221-237 (1991).

29. Y. Miyoshi, State of the Art in Pre-coated Steel Sheet for Automotive Body Materials in Japan, Iron Steel Inst. Japan International, Vol. 31, 1 (1991).

30. R. Baboian, Materials Performances, Vol. 24, 12 (1985).

31. S. Kurokawa, K. Yamato, T. Ichida, Automotive Corrosion and Protection, Proceedings of the CORROSION/91 Symposium, published by NACE 8-1 to 8-11 (1992).

32. S Fujita, H. Kajiyama, C. Kato, JEF GIHO, My, No.4, 8-14 (2004).

33. R. D. Granata, Automotive Corrosion and Protection, Proceedings of the CORROSION/91 Symposium, published by NACE 8-1 to 8-11 (1992).

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34. R. D Granata, in ‘Advances in Coating Technologies for Corrosion and Wear Resistant Coating’ Published by Mineral, Metal & Materials Society, 201-209 (1995).

35. H. Leidheiser Jr., Corrosion, Vol. 38, no. 7 374-383 (1982).

36. P.R. Roberge. Handbook of Corrosion Engineering, Mcgraw Hill Pubilication, New York, 339-344, (1999).

37. D.A. Jones, Principle and Prevention of Corrosion, 2nd_{edit., Prentice Hall}

Publication, New Jersey, 439-476 (1996).

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39. J.T. Johnson, Report FHWA-RD-01-156 Appendix N: Motor Vehicle, Corrosion Costs and Preventive Strategies in the United States, U.S. Department of Transportation Federal Highway Admistration (2000).

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43. H. Böhni, T. Suter, F. Assi, Sur. And Coatings Tech., Vol 130, 80-86 (2000) 44. T. Suter, A.Alkire, J. Electrochem. Soc. 148, B36 (2001).

45. F. Andreatta, Local Electrochemical Behavior of 7xxx Aluminium Alloys, Pasmans Offsetdrukkerij, Den Haag, 31-42 (2004.)

46. A.C. Bastos, Portugaliae Electrochemical Acta, 21, 371-387 (2003).

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48. H.S. Isaacs, J. B. Lumsden, A. Haughton, A. M. Shipley, H. Popenoe, E. Karplus, Corrosion and Protection of Light Metal Alloys, R. G. Buchheit, R. G. Kelly, B. A. Shaw, N. Missert Editors, The Electrochem. Proceedings Series (2003).

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This chapter was published as a scientific paper:

Y. M. Looi, J. R. Flores, C. Kwakernaak, J. H. W. de Wit, Materials and Corrosion, 55, No. 11, 831-836 (2004)

Chapter 3 Intergranular Corrosion on

Nd:YAG Laser-Welded

A653 Steel for

Automotive Application

Chapter 3:Intergranular Corrosion on Nd:YAG Laser-Welded A653 Steel for Automotive Application Chapter 4: Microstructure Characterisation of

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Synopsis

Laser welding techniques produce a narrower heat-affected zone than other conventional welding methods. However, laser welding is not exempt from high heat input during the welding process. This high heat input results in changes of the material properties including its corrosion behaviour; the formation of the heat affected zone increases the susceptibility of the material to intergranular attack. The residue of zinc at the weld due to condensation and splashing during the welding process may also adversely influence the corrosion behaviour of the material. The degree of susceptibility to corrosion strongly depends on the welding parameters. Electrochemical and microstructure characterisation were employed to study the influence of Nd:YAG (neodymium yttrium aluminium garnet) laser welding on an A653 galvanized steel at different welding parameters.

3.1 Introduction

Laser welding enables joining of materials with different thickness. This technique allows the application of subsequent press forming of the material without the need for further surface finishing [1-4]. One of the popular products from laser welding is the “laser-welded tailor blank” (LWTB). Nowadays, increasing numbers of car manufacturers use these blanks to produce light-weight fuel-efficient cars.

One of the factors influencing the quality of a blank is the heat-affected-zone (HAZ) situated adjacent to the fusion zone. The toughness and structural integrity of the welded material is to be seriously considered due to safety related issues. Poor toughness may result in fracture of the material, which may lead to catastrophic events [5]. The grain structure in the HAZ is coarser than that in all other areas. Even though laser welding is able to produce a narrower HAZ improving the toughness and integrity of the material, the corrosion susceptibility still remains high. In order to tackle this difficulty, many manufacturers start to use galvanized steel for their laser-welded products.

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Intergranular Corrosion on Nd:YAG Laser-Welded A653 Steel for Automotive Application

- 37 -

Zinc coatings are well known to enhance the lifetime of steels subjected to atmospheric or aqueous conditions by providing cathodic protection to the steel [6-8]. It is expected that the zinc layer on the surface will protect areas where the application of paint coatings does not provide the protection needed (e.g. at the interior of the automobile panels) [9]. However, when laser-welding is applied, the layer of zinc may be damaged. By controlling the welding parameters, such as welding speed and power source, the negative effects can be minimised. In other words, the size of the weld seam, the fusion zone and the HAZ (besides defects such as mismatching, half-penetration of weld, porosities, etc.) can subsequently be optimised to improve corrosion resistance of the joint. In this chapter, the influence of welding parameters from the electrochemical point of view is explored. Potentiodynamic tests, microstructural characterisation and chemical analysis provide a better understanding of the influence of laser welding on the corrosion properties of galvanised steel.

3.2 Experimental

Hot-dip galvanized steel sheet (A653) was supplied by CORUS in the Netherlands. The steel substrate was 1.2mm thick. The substrate was coated on both sides with an approximately 6.3µm zinc coating. Three types of samples were prepared and laser-welded by means of Nd:YAG laser. The welding parameters utilised are shown in Table 1 and the chemical composition of galvanized A653 steel are listed in Table 2.

Elements C Mn Si Al P Ti Cu Nb Sn Cr Ni Mo

Weight Percentage 0.001 0.161 0.020 0.033 0.008 <0.0001 0.011 <0.001 0.041 0.022 0.022 0.001

Specimens Welding Power Welding Speed

A653L 1750W 3.0m/min

A653MH 3000W 6.5m/min

A653ML 3000W 2.4m/min

Table1: Parameters of Nd:YAG laser welded galvanized A653 samples