Durability of polymer coated steel in diluted acetic acid environment

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Durability of polymer coated steel in

diluted acetic acid environment

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coating.

View on icefield ranges near the Hubbard glacier, Kluane National Park, Yukon Canada.

Photograph by Arie Korving

Title: Durability of polymer coated steel in diluted acetic acid environment

Proefschrift, Technische Universiteit Delft, Nederland Author: P.C.J. Beentjes

ISBN: 90-805661-4-4

Copyright: Corus Technology BV

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diluted acetic acid environment

Duurzaamheid van met kunststof bekleed

staal in verdund azijnzuurmilieu

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 13 april 2004 om 15.30 uur

door

Petrus Cornelis Jozef BEENTJES doctorandus in de scheikunde

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Prof. dr. J.H.W. de Wit

Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof. dr. J.H.W. de Wit Technische Universiteit Delft, promotor Prof. dr. R. Boom Technische Universiteit Delft

Prof. dr. ir. S. Radelaar Netherlands Institute for Metals Research Prof. dr. J. Reedijk Universiteit Leiden

Prof. dr. H.Terryn Technische Universiteit Delft Prof. dr. ir. W. J. van Ooij University of Cincinnati

Dr. G. Grundmeier Max-Planck-Institut für Eisenforschung

The research in this thesis was sponsored by Corus RD&T and carried out under the project number MC6.00076 of the Strategic Research Program of the Netherlands Institute for Metals Research (NIMR).

ISBN: 90-805661-4-4 Printed in the Netherlands

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Chapter 1 General Introduction

Summary

This work aims at the replacement of the metallic chromium layer that is applied on steel used for packaging purposes prior to application of a thermoplastic polymer coating. To this purpose we need to obtain a clear view as to why the metallic chromium coating of only 10 nanometers thickness is so important for adhesion and corrosion protection.

1.1 Polymer

coated

steel

Carbon steel not provided with any coating is not well protected against common (wet, acidic and/or salty) environments and will then corrode readily. For this reason, steel is normally supplied to the end user with either a metallic or organic coating or both. As steel substrates are usually formed and shaped into products, one has to decide whether application of the coatings should take place before or after forming of the steel. For every market segment a specific choice has been made as regards the coating technology, based on the economy of the processes, the legislative aspects concerning environmental protection and safety and on technical possibilities. Packaging steel, mostly used for the production of food or beverage containers and ends, is normally supplied with a layer of tin of between 1.5 to 11 gr/m2. Most economically, this layer is applied by the steel manufacturer, i.e. before deformation into cans. The tin layer provides corrosion resistance in certain environments and acts as a very good lubricant during forming processes like draw and wall ironing into cans. To give a better protection and appearance, the tin coated substrate is usually lacquered by the canmaker before forming (in case of deep drawing only) or after forming (in case of the very demanding wall ironing process). Therefore, packaging steel is normally coated a number of times (metallic coating, organic basecoat, topcoat, printing inks, varnishes), before shipment of filled cans to the retailer. Figure 1.1. shows examples of cans made by the Draw Redraw (DRD) process and cans made by the Draw and Wall Ironing (DWI or D&I) process. These processes will be explained in more detail in section 2.5.

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Figure 1.1. DRD- and DWI cans.

In order to reduce the number of process steps in the supply chain, and to reduce environmental problems like organic volatiles emissions, Hoogovens started an investigation into combining most of the coating processes into one continuous steel strip line about 10 years ago. The product, that is currently commercially available under the trade name Protact®_{, consists of a} chromium coating, much thinner than the original tin coating, combined with a polymer coating, applied through extrusion coating of either PET (poly(ethylene terephthalate)), or PP (polypropylene). The chromium coating is applied electrolytically onto steel strip, using a chromate containing electrolyte bath. The electro deposition process produces a very thin metallic chromium coating (typical thickness of 10 nm) with a thin chromium(III) oxide layer (about 4-7 nm) on top (ECCS: Electrolytically Chromium Coated Steel). Although the deposition process is continuous and well controlled and the product does not contain any Cr(VI), the use of chromates is cause for some concern, as the electrolytic chromium coating process is becoming more and more expensive as a result of restrictions imposed by governments on the use of (carcinogenic) chromates and on the treatment of chromate containing waste (for instance waste water after rinsing).

Although PET-coated ECCS is able to withstand the rigours of the wall ironing and necking processes and is able to protect the can contents for a very long time, it behaves less ideally in low pH environments containing acetic acid. In this environment corrosion of the steel substrate, especially in heavily deformed areas, can proceed quite rapidly causing uptake of iron by the can content. One of the aims of this Thesis is to find out what is so special about acetic acid.

1.2 Research

aim

This Thesis has more than one objective. From a scientific point of view it is interesting to develop a more fundamental understanding of the properties of the metallic chromium coating and some fundamental knowledge of adhesion between metal(oxides) and organic coatings. From an industrial point of view,

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it is interesting to develop new processes that avoid the use of (heavy) metal coatings altogether and that can be literally inserted into an existing production line with accompanying demands on process time (a few seconds), economy and safety (no organic solvents allowed).

When testing Protact, the most severe can content environment encountered seems to be acetic acid, which is critical to the product. As an example, Figure 1.2 shows the result of exposure of a PET coated ECCS DRD can to glacial acetic acid vapor (glacial acetic acid meaning pure acetic acid, melting point 17°C) at room temperature during 24 hours. This picture gives a view of the inside wall- and bottom of the can (after cuting it open). Normally, cans contain acetic acid in a very low concentration in water (less then 3%), so obviously exposure to glacial acetic acid vapor is not a good simulation of reality. Still, this experiment clearly shows the difference in behaviour between non-deformed and strongly deformed parts of either DRD- or D&I-cans. The can bottom did not corrode, except on the terraces, whereas the can wall turned out to be heavily corroded, particularly in the upper part. After the first deep drawing operation into a cup, part of the material, forming the flat cup bottom, remained unaffected. After exposure of the DRD can to acetic acid, the borderline between the first draw cup bottom and first draw cup wall showed up clearly. The transition is often called a witness mark.

Figure 1.2. Severe corrosion on DRD can wall after exposure to vapor of glacial acetic acid during 24 hours at room temperature.

Acetic acid turns out to be a very versatile molecule and plays the leading role throughout this Thesis.

Chapter 2 gives a description of the polymer coated steel product and its properties toward deformation, water uptake and corrosion in an acetic acid environment. Although organic coatings, and polyesters in particular, are usually strong barriers to electrolyte, the acetic acid molecule is able to diffuse through organic coatings towards the metal interface just like many organic solvents.

terrace

witness mark

terrace

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Chapter 3 deals with the chemistry of silane compounds. The acetic acid moleucle is encountered here as a catalyst for the hydrolysis of silanes.

In Chapter 4 the cleaning aspects of metal surfaces are investigated, showing that metal surfaces are usually covered with mono- or multi-layers of carboxylate species resembling acetic acid molecules.

Chapters 4 and 5 deal with silanes that are able to form siloxane layers on metal substrates. However, chapter 5 shows that acetic acid, used to hydrolyze silanes, is able to interfere during the adsorption stage of silanols. Chapter 6 shows that polymers or monomers containing acid groups may be of some use to enhance bonding strength to metal substrates. In this Chapter it is shown that the application of an intermediate metallic layer between polymer and steel provides the opportunity to prove that acetic acid attacks by dissolving the iron oxide layer contacting the steel substrate.

Finally, in Chapter 7, a theory is presented proclaiming that the most important property of an organic coating with respect to corrosion protection is its ability not to form voids during its lifetime, thereby prohibiting formation of a condensed water phase, capable of dissolving ions.

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Chapter 2 Properties of polymer coated packaging steel

Summary

The main function of a polymer coating on a metal substrate, as far as corrosion is concerned, is the production of an environment on the metal surface with extremely high resistance to movement of ions. In a polymer matrix, ions are able to move only in case of complete hydration. For optimum protection of the metal, the polymer coating must maintain its density on a molecular scale especially close to and at the interface with the metal so as to impede the mobility of hydrated ions. Plastic deformation of the product caused by forming processes followed by sterilization treatment, leads to development of flaws in the coating and fissures between the polymer and the metal substrate that, even on a molecular scale (less than a nanometer) lead to prospects for water to solvate ions.

Unlike ionic species, acetic acid is able to diffuse through organic coatings in molecular form, forming ions in the presence of sufficient amounts of water only. This leads to acidification at spots with insufficient polymer density, for instance in molecular scale fissures between the organic coating and the metal substrate.

Organic coatings bond to “metals” through an intermediate oxide layer, usually through acid-base interactions and hydrogen bonds. It is the concentration of these bonds, together with the resistance against hydrolysis that determines the rate of diffusion of water and hydrated ions along the metal surface.

2.1 Introduction

All layers of polymer-coated steel contribute to the performance of the product, one way or another. This Chapter deals with the properties of each one of them and the interactions between the layers, in order to obtain a basic understanding of what may happen in a corrosive environment.

The last section discusses some tests to measure adhesion and durability of the coating system, and discusses the reasons for choosing the one test to be used throughout this Thesis: the acetic acid undermining test.

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2.2 Properties of the organic coating

2.2.1 The PET chain and modifications

Poly(ethylene terephthalate) (PET) is a thermoplastic material that, due to its favorable properties and declining price, is a strong competitor to plastics like polyolefins and polystyrene, to other materials like metals (beverage packaging) and natural fibers (textile, carpeting). It is also the material of choice for polymer coated metals (steel and aluminium) for packaging purposes, due to its formability, abrasion resistance and its water content, which is low compared to other polymer materials like polyamides.

There is a big market for PET films of high quality for applications in the audio and video sector and for packaging purposes. The availability of these films made it possible for steel companies about fifteen years ago to start producing PET laminated packaging steel based on ECCS.

The water uptake and the barrier properties of PET, especially at high temperatures (80 –120°C), depend on the crystallinity and crystallization rate during (hot) water uptake. The rate of crystallization and the melting point of PET can be controlled by introducing monomers that lower the chain symmetry, resulting in a lower crystallization rate and melting point. PET is made by polymerizing glycol with either terephthalic acid or dimethyl terephthalate at a high temperature in the presence of catalysts (Figure 2.1).

Figure 2.1. Condensation reaction between terephthalic acid and glycol producing PET and low molecular weight compounds, and the backward reaction (hydrolysis).

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O HO O OH terephthalic acid or O CH₃O _O OCH₃ dimethyl terephthalate + OH HO glycol O O O + water or methanol O PET n

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The condensation type reaction produces volatile molecules like water or methanol. It is very important to understand that this reaction is reversible, causing bond breaking and forming at random during the extrusion stage, catalyzed by traces of water. Statistical processes control the molecular weight distribution. It is possible, by introducing co-monomers (dialcohols, diacids or cyclic anhydrides) during the polymerization stage or during the extrusion stage that build in into the polymer chains at random, to influence the crystallization rate or other properties (Tg, gas barrier, average molecular weight) of the polymer. Part of the terephthalic acid reactant may be replaced by isophthtalic acid, or part of the glycol reactant may be replaced by diethylene glycol (usually unintentionally: by condensation of two glycol molecules during the polymerization step) or by trans-1,4-cyclohexanedimethanol (CHDM). The melting point of PET changes dramatically on modification with CHDM. At zero percent modification (pure PET) Tm is 255°C, whereas at 100% modification (all glycol monomers are substituted by CHDM) Tm increases to 298°C. However, mixing glycol and CHDM results in random copolymers having lower crystallization rate and lower Tm. In fact, at certain compositions the crystallization rate falls to zero and a crystalline state is never reached. This type of amorphous PET is called PETG (PET glycol modified).

Preliminary studies to apply PET to steel substrates on a small scale were not successful. PET does not dissolve readily in “normal” solvents that don’t attack steel substrates (PET dissolves in strong Lewis acids like p-chloro phenol that is a severe etchant). However, the ease with which PETG dissolves in chloroform and the fact that it does not crystallize during any heat treatment makes it the ideal model compound for tests throughout this work. PETG has a Tg of 75-80°C and it has no Tm.

Chain scission and the subsequent formation of new bonds provides the opportunity to attach PET(G) covalently to other materials. It is possible to form covalent bonds between the PET polymer chain and a surface containing primary amino groups through aminolysis(98,99,100)_{. However aminolysis, like} hydrolysis, causes strong molecular weight reduction, as every reacting molecule causes chain scission(101)_{. Therefore the concentration of amino} groups at the substrate surface should be kept very low; if not, a low molecular weight hydrophilic weak boundary layer is formed between the polymer and the substrate(104,105)_.

One way to avoid chain scission is to use the reactivity of the PET end groups. Glycol end groups are able to react with isocyanates through addition(103)_. Carboxylic acid end groups are able to react with epoxy groups attached to the substrate surface through oxirane ring opening(106)_{. The addition reaction} does not lead to chain scission.

Xue(107)_{attached γ-glycidoxypropyltrimethoxysilane (GPS) covalently to PET} chains (and model monomers) through addition to the carboxylic acid end group (Figure 2.2), to obtain better adhesion to glass fiber.

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Figure 2.2. Reaction taking place between carboxylic acid terminated polyester and γ-glycidoxypropyltrimethoxysilane (GPS) (107)_.

All efforts to link PET covalently to substrates of other materials have the disadvantage that it is not possible to form a high concentration of covalent bonds. Trans-esterification reactions (aminolysis) lead to low molecular weight material and therefore should be kept to a minimum, whereas addition through chain ends produces covalent bonds at very low concentrations (PET molecular weight (Mn) is 26000). In Chapter 5 the experience obtained with the silane compound GPS for improvement of adhesion of PETG to siloxane coated steel is reported.

2.2.2 Diffusion of water and acetic acid in PET

For corrosion processes of polymer coated metal cans, the rate of transport (diffusion) of water, acid groups containing organic molecules and/or ionic species to the polymer-metal interface is important. When cooled to below their glass transition temperature (Tg), all polymers contain excess free volume by virtue of existing in a nonequilibrium state. Amorphous regions have a lower density than crystalline regions in the same material, due to stacking of molecular chains being more chaotic(1)_{. Freedom of movement of} chain segments and small molecules present in this region is high compared to the crystalline region. Polymers are permeable because the dissolved small molecules are able to diffuse through the amorphous phase of the polymer. The excess free volume is not uniformly distributed on a molecular scale, but rather clustered into tiny voids or cavities(2)_{. Small molecule penetrants} adsorb onto the walls of these cavities. Consequently, when a glassy polymer absorbs a gas or other small molecule, the penetrant can exists in two ways:

• uniformly dissolved throughout the amorphous regions of the polymer, and PET chain OH O

+

O O Si O O O C H3 CH3 CH3 PET chain O O OH O Si O O O C H3 CH3 CH3 100°C 30 min

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• adsorbed onto the walls of the excess free volume cavities.

As during the Protact process the polymer coating is cooled very fast (from 280 to 20°C in less than half a second), PET is quenched in its amorphous state, and its free volume is high. PET does not crystallize at room temperature, about 55 degrees below the Tg of the dry polymer.

The equilibrium amount of water absorbed by PET is an almost linear function of the relative humidity, about 0.5% at 50% rel. hum. to 1% at 100% rel. hum at room temperature(2)_{. The diffusion coefficient of water in} amorphous PET was found to be D = 3.94x10-9_cm2_/s(120,121)_{and D = 4.5x10}-9 cm2_/s(2)_{. Diffusion of water into PET at room temperature is depicted in} Figure 2.3, which shows the calculated water uptake by an initially dry and amorphous sheet of PET of thickness 0.635 mm and diffusion coefficient D = 4.5x10-9_cm2_{/s at 23°C.}

Figure 2.3. Water concentration profile in an initially dry 0.635 mm thick amorphous PET sheet after dipping in water at 23°C for different periods(2)_. Even after an exposure time of 8 hours only a very low concentration of water exists at the centre of the sheet. On the other hand, Figure 2.3 shows that a 20 micron PET coating on steel, dipped in water at 23°C, becomes fully wet after a short amount of time (some minutes only).

At higher temperatures, especially above Tg, diffusion rates are much higher. This means that the steel-polymer interface of a polymer coated steel can is usually in equilibrium with the amount of water in the product packed, and that PET at a thickness of 20 micron may be considered to be a membrane, and not a barrier to water(10)_.

Like most low molecular weight compounds added to a polymer, water plasticizes PET: the Tg of PET, stored at 100% rel. hum is 16°C lower (63°C) than the Tg of dry PET (79°C). During forming or (sterilization) heat treatments the physical and mechanical properties of the coating are

-1.0 0.0 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Relative position (surfaces at –1 and 1, center at 0) Normalized Water Concentration 24 hrs 8 hrs 1.3 hrs 0.3 hrs

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dependent on (T-Tg), and lowering Tg by some amount has the same quantitative effect as increasing T.

Organic solvents penetrate PET to a much larger extent than does water(3,4,5,6,8)_{. Even solvents that contain acid groups like acetic acid and} hydrogen sulphide are able to penetrate polymers. The acetic acid molecule dimerizes readily, depending on concentration. In non-polar liquids or polymers, or in the vapor phase, intermolecular hydrogen bonds between two acetic acid molecules are favored, resulting in the formation of cyclic dimers. In aqueous solutions, acetic acid monomers are predominant in very diluted solution, and the formation of linear and cyclic dimers is favored at increased acetic acid concentration (Figure 2.4).

This self-association can be shown with the help of FTIR-ATR equipment (carbonyl stretching frequencies)(6,7)_.

Dutheillet(3)_{studied the ingress of acetic acid and water-acetic acid mixtures} into an epoxy matrix. Whereas water has a low solubility in epoxy resin (< 1%), acetic acid is highly soluble (more than 40% mass uptake). Acting as a swelling agent, acetic acid enhanced water solubility in epoxy matrices. Contrary to organic molecules with carboxylic acid groups (formic acid, acetic acid, lactic acid) that can hide their dipole and their hydrogen bonding nature into dimer structures, the solubility and rate of diffusion of mineral acids (sulphuric and nitric acid), that are fully dissociated in aqueous solution, is very low(9)_.

Figure 2.4. Possible structures of acetic acid: (A) monomer (B) cyclic dimer (C) linear polymer.

2.3 The organic-inorganic interphasial region

2.3.1 The oxide layer

The rate of corrosion of most technologically important metals in air and in many aqueous environments is slowed down by a “passive” oxide film

C H₃ OH O C H3 OH O CH3 O H O C H₃ OH O CH3 O H O ... ... ...

A

_B

C

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films has led to widespread investigation of their structure and chemistry. The performance with respect to corrosion and adhesion of coated and bonded metal substrates relies heavily upon the composition of this layer. Metals like aluminium and titanium are readily oxidized in the presence of oxygen and in water, but the oxide layers formed on the surface prevent diffusion of both electrons and ionic species strongly(19)_{. For this reason,} corrosion is prevented very effectively after reaching an oxide layer thickness of about 5 nanometers. These thin films reform immediately in case of damage in the presence of either water or oxygen, and then continue to protect the metal from corrosion. In water a thicker and more permeable layer of hydrated oxide usually forms(25)_{. When Al is immersed in water at 100°C} and 1 atmosphere pressure a surface layer of pseudoboehmite (which is poorly crystallized boehmite, AlOOH) forms(18)_{, whereas exposure to liquid} water between 20 and 90°C leads to loosely bound bayerite crystals (aluminium hydroxide, Al(OH)3). The next section will deal with the importance of hydroxyl groups to adhesion of organic coatings.

In the presence of oxygen, the oxide layer on chromium metal mainly grows by outward migration of Cr3+_-ions(122,136-140)_{. These ions are created at the} Cr/Cr2O3 interface by electron transfer from Cr atoms to dissociatively adsorbing oxygen at the oxide surface(124)_{. The electrons can migrate through} the oxide film because Cr2O3 has a high defect conductivity(23,37,128). The electron transfer to oxygen creates an electric field normal to the surface. The outward diffusion of the Cr3+_{-ions, mainly along grain boundaries in the} oxide film, is driven by this electric field(122)_{. Maurice}(125)_{investigated the} growth, thickness, composition and structure of chromium oxide thin films formed by exposing Cr(110) single-crystal surfaces to gaseous oxygen at 25 and 350°C in a vacuum. At 25°C, a granular and non-crystalline oxide was formed, growing with a constant ∼ Cr2O3 stoichiometry up to a limiting thickness of 0.9 nm (i.e. only four mono-layers of Cr2O3). Due to inevitable trace amounts of water, the Cr2O3 film was hydrated, i.e. some hydroxide was incorporated into the structure. The stoichiometry changed from (Cr2O3 + 2CrOOH) to (Cr2O3 + CrOOH) on annealing to 350°C. The high concentration of defects and grain boundaries in this film led to uniform film thickening until the limiting oxide thickness. Further thickening at 350°C by layer-to-layer growth leads to formation of an oxide layer-to-layer remaining non-crystalline with limiting thickness of 4.6 nm. Above a transition temperature of 450°C the concentration of migration paths for cations becomes lower, resulting in roughening of the oxide surface(122,123)_.

Cr(III) migration is almost always rate determining for oxide growth, even at oxygen partial pressures as low as 6.5 x 10–5_mbar(122,123)_{. If the oxide layer is} damaged, local breakdown of passivity occurs, but repassivation of the defect is extremely fast.

The mechanism of oxide formation on chromium in water is more complex. From rotating ring disc experiments Haupt et al(21)_{concluded that chromium} dissolves as Cr2+_{. Okuyama}(22)_{studied the dissolution and passivation of} chromium in water at different pH values and found no dependence of the

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dissolution rate on pH. The following dissolution and passivation mechanism was proposed: Dissolution: Cr + H2O Cr(I)OHad + H+ + e -Cr(I)OHad Cr(II)OH+ + e -Cr(II)OH+_{+ H}+ Cr2+_{+ H}₂_O Passivation: Cr(I)OHad + H2O CrO(OH) + 2H+ + 2e

-The passivation reaction of chromium was proposed to be due to the formation of amorphous chromium trivalent oxide CrO(OH) (obviously, formation of Cr2O3 by the reaction: 2Cr(I)OHad + H2O Cr2O3 + 4H+ + 4e- is another possibility).

In nitrogen-purged acid solutions, the hydrogen evolution reaction plays a major role(23)_{. Gerretsen}(129,133-135)_{combined Electrochemical Impedance} Spectroscopy (EIS) measurements with Cyclic Voltammetry (CV) and proved that hydrogen adsorbs on the surface and is even stored in the metal and in the passive film. Dobbelaar(23,130-132)_{proposed the following mechanism for} chromium dissolution and passivation in nitrogen purged acid solutions: Hydrogen evolution:

H+_{+ e}- H_ad,Cr ½ H₂_{(slow) and} H+_{+ e}- H_ad,Ox ½ H₂_(fast)

Direct dissolution of chromium atoms from active sites: Cr Crad+ + e- Crsol2+ + e

-in which Crad+ means Cr(I) still adsorbed to the metal, and Crsol2+ means Cr(II)-ions hydrated by electrolyte.

Passivation:

Cr + nH2O Cr(Ox) + yH+ + ye-

Since the exact composition of the oxide film was dependent on the potential applied(134,135)_{, it was represented as Cr(Ox).}

Maurice(141)_{, using a combination of X-ray photoelectron spectroscopy (XPS)} and scanning tunneling microscopy (STM) on passive films formed on Cr(110) single-crystal surfaces in 0.5 M H2SO4 showed the existence of ordered domains (nanocrystals of oxide) emerging at or near the film surface, in a matrix of chromium hydroxide without structural periodicity (a vitreous

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minimize the density of crystalline defects such as grain boundaries between nanocrystals, and thus would improve the resistance to passivity breakdown. In contrast to the passive layers on chromium and aluminium the passive layers on steel are quite complex, containing two oxidation states (Fe(II) (ferrous) and Fe(III) (ferric)) and two crystalline structures. Determination of relative Fe(II) and Fe(III) concentrations and the structure of the layer is difficult. The stoichiometry of the oxide layer on iron is not completely like any of the pure anhydrous oxides, like wüstite (FeO), hematite (α-Fe2O3), maghemite (γ-Fe2O3), lepidocrocite (γ-FeOOH), goethite (α-FeOOH), or magnetite (Fe3O4)(33). The Fe(III)/Fe(II) ratio depends on a number of factors, for instance the electrochemical history of the sample and the presence of oxidizing agents like oxygen. The stoichiometry of the oxide, presented by the formula Fe3-δO4, ranges from δ = 0 at the metal/oxide interface (meaning Fe3O4 which has a cubic structure containing twice as many ferric as ferrous ions) to δ = 1/3 at the oxide/air interface (meaning Fe2O3, and having a hexagonal structure unrelated to that of the cubic oxides(81,83)_{. Due to the} misfit between these two crystal structures, the mechanical strength of the oxide becomes impaired.

The mechanism of dissolution of the iron oxide film has been shown to be associated with valence changes within the oxide film(12,13,14,15)_{. This} mechanism will be considered in more detail in Chapters 4 and 6. Cahan and Chen(13,14) _{characterize the chemical composition of the oxide layer loosely as} “a highly protonated, tri-valent iron oxy-hydroxide capable of existing over a relatively wide range of stoichiometry”. The total thickness of the natural oxide film on iron formed in dry air is in the order of 3 nanometers(32)_.

The passivation behaviour of stainless steels has been investigated by numerous authors(27,28,29,30)_{. One of the best-known facts in corrosion science is} that stainless steels require about 13 at% (12 wt.%) Cr to become truly “stainless”, i.e. resistant to rusting in moist atmospheres or dilute aqueous solutions. On Fe1-xCrx alloys with x < 0.12 the oxide films are relatively thick. They may be porous, and there may be a bottom layer (maghemite (γ-Fe2O3) with some of the Fe3+_{replaced by Cr}3+_{) offering some degree of protection for} the underlying alloy, and an outer hydrated part (hydroxides). On alloys with x > 0.12 the films are thin, compact and enriched in Cr2O3 in their molecular layers next to the metal surface, while on top of the relevant barrier layer one again finds chromium hydroxides. The strong enrichment of the oxide layer in Cr2O3 provides the corrosion resistance of stainless steel. While iron (hydr)oxides dissolve preferentially, chromium oxyhydroxide is left behind. The higher the amount of chromium in the bulk of the material, the thinner the passive layer at the surface becomes, showing the effectiveness of the chromium oxide barrier at higher Cr-content.

The potential-pH diagrams made by Pourbaix(144)_{show that both iron and} chromium are passive in alkaline solution, whereas aluminium dissolves at

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high pH. Alkaline degreasing treatments will turn out to be effective for steel and ECCS (Chapter 4).

2.3.2 Surface hydroxyls

Cleavage of a metal oxide in high vacuum produces a fresh surface with both metal cations and oxygen anions lying bare. These metal cations, being coordinatively unsaturated, strongly react with any electron donating molecule, like water. The electron donating molecules are adsorbed, either molecularly (by association) or by dissociation. Under normal circumstances therefore, the outermost surface of an oxide (the first atomic layer) consists of oxygen atoms (bridging oxide or hydroxyl groups) (Figure 2.5). Bowden and Throssell(43,64)_{found that, for smooth Al, Fe and SiO}₂_{surfaces, at ambient} temperature and relative humidity, there is always some physisorption of water molecules, and no coordinatively unsaturated cations remain at the surface(143)_{. The number of hydroxyl groups per unit surface area depends on} oxide composition, level and type of impurities, prior thermal history, time and temperature of exposure to water vapor, oxide crystallinity and particle size. Under ambient conditions, a significant fraction of the metal atoms at the surface exist as –MOH groups (Figure 2.5), and even at very high temperatures and at high vacuum some hydroxyls remain at the surface.

Figure 2.5. A schematic representation of a metal oxide surface, (A) just after cleavage of the oxide in high vacuum conditions (M at the surface is coordinatively unsaturated), (B) after chemisorption of a small amount of water, (C) after physisorption of water.

.

O M M M M O O O O O O O O O O O M M M M O O O O O O O O O O

A

H

₂

O

B

O O O O M M M M O O O O O O O O O O O H H O H H H H H O O O O M M M M O O O O O O O O O O O H H O H H O H H O H H H H H

C

∆

H

₂

O

OH OH OH O M M M M O O O O O O O O O O OH OH OH O M M M M O O O O O O O O O O OH OH O M M M M O O O O O O O O O O

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Organic coatings on a metal oxide either bond to the hydroxyl groups, producing hydrogen bonds, or to the cationic site (M) after removal of the hydroxyl group, producing coordinating bonds. Both types of bonds will be discussed in this Thesis.

2.3.3 Adhesion of organic coatings

The literature on intermolecular and inter-atomic forces is extensive(59,60,61,62,63,64)_{. With respect to the type of interactions, the} intermolecular and inter-atomic bonds are classified as illustrated in Table 1. Table 1. Typical inter-atomic and intra-molecular forces(60)

There are many different types of forces involved in the interfacial region, among which the hydrogen bond (or, in a more general sense, the acid-base interaction) and coordinating interactions are the most important ones for adhesion to high-energy metal oxides. Bolger(61) _{provided an extensive review} concerning hydrogen bonds. At a later stage, the role of coordinate bonds was emphasized.

2.3.3.1 The

hydrogen

bond

To most chemists, the idea that compounds having ester groups or other “basic” groups need “acidic” solvents to dissolve these compounds comes naturally. The reason for this is, that one imagines a strong interaction taking place between the lone pairs of a ketone, ether or ester structure and the

Type of force Range (nm) Energy (kJ/mole)

I. Interatomic bonds

(a) Ionic 0.15-0.24 335 - 1050

hydrogen 0.26-0.30 8 - 42

(b) Covalent 0.15-0.24 63 - 351a

(c) Metallic 0.26-0.30 110 - 350

II. Intermolecular bonds (indefinite range)

(a) Dipole-Dipole (excluding H-bonds) 4 - 21

(b) London (dispersion) 4 - 42

(c) Dipole-Induced Dipole 2

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electron deficient hydrogen of the acidic solvent, creating a hydrogen bond(75,76)_:

The chloroform molecule has the ability of being a proton donor, whereas the carbonyl group is a proton acceptor, exclusively. A hydrogen bond is formed by an atom of hydrogen strongly attracted to two atoms rather than one such that it may be considered as acting as a bond (or bridge) between them. Since hydrogen, having only one (1s) orbital, can form only one covalent bond, the hydrogen bond is always partially ionic in character and can be formed only between the most electronegative elements, F, O, N and Cl(66)_.

Extension to non-hydrogen bond interactions is facilitated by the Lewis acid-base terminology, in which acids are defined as electron acceptors, able to accept electrons into their low energy lowest unoccupied molecular orbital (LUMO), and bases as electron donors, able to share electrons from their highest occupied molecular orbital (HOMO) with an acid(65)_:

A (acid) + :B (base) A:B

Hydrogen bonding constitutes a subclass of Lewis acid-base bonding because of the presence of a proton in the bond structure.

Molecules possessing a hydroxyl group are able to act both as a Lewis acid and as a Lewis base, depending on the availability of acidic or basic compounds to interact with.

Hydrated surfaces adsorb water moecules through H-bond interactions wherein the surface acts either as the acid or base:

It all depends on the electron density on the =MOH oxygen atom which is linked to the electronegativity (and thus valence and size) of the M cation. On altering the pH of the aqueous phase, the surface acquires an ionic charge through: H Cl Cl Cl R O R

=MOH

….

OH

₂

=MOH + H

₂

O

=MO

….

HOH

H

as acid

as base

=MOH

₂….

OH

₂

=MOH + H

₂

O

OH

=MO

- ….

HOH

-H

+

(25)

(Only a small amount of the surface sites acquire a charge).

For every oxide, there exists some pH at which the number of positive charges equals the number of negative charges. This pH value is defined as the IEPS, the isoelectric point of the surface. A high IEPS value indicates a basic surface.

As the positive charge presence in the acidic environment and the negative charge presence in the alkaline environment cause an increased attraction of water dipoles with the oxide layer, the minimum oxide-water interaction occurs toward the iso-electric point, i.e. the wettability is at its lowest.

IEPS values with some relevance for this Thesis are shown in Table 2. For a given metal oxide such as iron or aluminium the IEPS can vary over a relatively broad range, reflecting the composition and crystalline (or amorphous) form of the oxide, impurities, degree of hydration and the state of oxidation or reduction (valence). In case of highly charged cations present in the oxide (M of valence +3 or higher), the surface hydroxyl groups tend to be more acidic (this is due to the increased delocalization of oxygen negative charge). Therefore the IEPS is highest for the divalent metals (Mg, Ca, Fe(II), Ni etc), in the medium pH range for most hydrated trivalent metal oxides, and is lowest (most acidic) for metals of valence 4 and higher. The low IEPS of silica reflects the high acidity of the silanol group that is due the strongly electrophilic character of the silicon atom in SiO2. It is important to know that oxidizing treatments of steel surfaces, like a chromic acid etch, hydrogen peroxide or anodic alkaline cleaning treatment, increase the valence of the surface metal ions, reducing the IEPS to a less basic range.

Table 2. Iso-electric points for solid oxides in water(44)

oxide Iso

Electric

Points

dry rinsed in water

AlOOH (Boehmite) 6.5-8.8 Amorphous Al2O3 6.7 9.2 Al(OH)3 (Bayerite) 9.3 Fe(OH)2 11.5-12.5 Fe2O3 6.5-6.9 (800°C calcined) 8.5 FeOOH 6.5-6.9 (105°C) 8.5 Fe3O4 6.3-6.7 8.6 Cr2O3 7.0-7.2 SiO2 gels 1.8

From the above, it follows that an oxide surface can form hydrogen bonds with polar groups by a variety of mechanisms. Either the surface hydroxyl or the organic group can act as the acid (electron acceptor), or as the base (electron donor), forming a hydrogen bond that is more or less ionic.

Bolger(61)_{distinguished two general interaction types. For type A interactions} the surface provides the basic group that interacts with an organic acid. For

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type B interactions the surface provides the acidic group that interacts with an organic base. Within each type, two extreme conditions in which the interaction is predominantly ionic or non-ionic are distinguished.

To predict the probability of the interaction of a given oxide surface-polar group combination, the equilibrium constants for type A or type B interactions may be expressed as:

where HX is an organic acid for type A reactions, and X is a base for type B reactions. To derive ∆A and ∆B the surface reactions may be rewritten as: MO-_{+ H}+ MOH K₁_{= [MOH] / [MO}-_].[H+_]

MOH + H+ MOH₂₊ _K₂_{= [MOH}₂₊_{] / [MOH].[H}+_] and for

MO-_{+ 2H}+ MOH₂₊ _K₁_.K₂_{= [MOH}₂₊_{] / [MO}-_].[H+_]2

At the isoelectric point, [MOH2+] = [MO-], and K1.K2 = (10 – IEPS)-2. (IEPS meaning here the pH value at the isoelectric point).

Assuming symmetry around the IEPS gives K = K = (10 – (IEPS)₎-1_.

=MOH + R-C-O-R

=MO

….

C=O Type A-1

=MOH + R-C-OH

=MOH

₂

O-C Type A-2

=MOH + R-C-O-R

=MOH

….

O=C Type B-1

=MOH + NH

₂

-R

=MO NH

₃

–R Type B-2

O

H

OR

R

O

+

-O

O

OR

R

-

+

κ

_A

=

[MOH2+][X-]

[MOH][HX]

κ

_B

=

[MO-][HX+]

[MOH][X]

∆

_A

log

κ

_A

∆

log

κ

_B

κ

_A

=

[MOH2+][X-]

[MOH][HX]

∆

it follows:

and, using the same substitutions: ∆B = pKA(B) – IEPS.

If ∆A or ∆B are positive, the ionic reactions should predominate (type A-2 or B-2), whereas if ∆A or ∆B are negative, the dipole interactions should dominate (type A-1 or B-1). At very negative ∆A or ∆B, ionic interactions are negligible and dipole interactions are weak.

Apolar polymers like polyethylene, which can be regarded as both a very weak acid and a very weak base (pKA(A) large and negative and pKA(B) large and positive), give large negative ∆A or ∆B values, and as such adhere to interfaces only through weak van der Waals forces. Table 3 contains some values of pKA(A) and pKA(B) of some organic acids and bases, and the ∆A and ∆B values on substrates with IEPS is 2 (acidic, like SiO2), 8 (slightly basic, Al2O3 or Fe2O3) and 12 (basic, MgO). There are only a few positive ∆A and ∆B values, representing strong, predominantly ionic, hydrogen bonds. There is a big difference between the iso-electric points of silicon dioxide and common metal oxides, and as a result, the interactions with adsorbing molecules differ. Table 3 predicts that the strongest hydrogen bonds between a coating and a metal oxide are produced by acidic groups.

These bonds will not be hydrolyzed by water as such, because hydrogen bonds formed by water are always primarily non-ionic, i.e. not very strong. Figure 2.6 shows the dominant interaction modes and relative bond strengths as predicted by Bolger and Michaels(61)_.

∆

_A

log

κ

_A

= log K

₂

+ log K

_A(A)

= IEPS – pK

_A(A) acid: HX H+_{+ X}- _K A(A) [H+_][X-_] [HX] base: HX+_{+ H} 2O X +H3O+ KA(B) [X][H+_] [HX+_]

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Table 3. pKA(A) and pKA(B) of some organic acids and bases(61). Iso Electric Points

SiO2 Al2O3 MgO SiO2 Al2O3 MgO

2 8 12 2 8 12

organic acid pKA(A) pKA(B)

∆A ∆B primary amines 20 10 -18 -12 -8 +8 +2 -2 monocarboxylic acids 4.5 -6 -2.5 +3.5 +7.5 -8 -14 -18 maleic acid(64)_1.83 _{+0.17 +6.17} _+10.17 water 15.7 -1.7 -13.7 -7.7 -3.7 -3.7 -9.7 -13.7 benzosulphonic acid(64) 0.7 +1.3 +7.3 +11.3 acetone 20 -7.2 -18 -12 -8 -9.2 -15.2 -19.2 phenol(64)_9.89_-7.89_-1.89_+2.11 o-silicic acid(64)_9.66 _{-7.66 -1.66 +2.34}

Figure 2.6. Dipole orientations, dominant interaction modes, and relative strength of hydrogen bonds for high IEPS surface (for example, MgO) and low IEPS surface (for example, SiO2).

Unfortunately, de-ionized water is not the only species attacking the interface

Si O N H R R Si O H H O C

=

O R H Si O H O H H Si O H Si O H H O Si O H O C R R

Aliph. amines >> Carb. acids = Water > Phenol > Ketones

∆B= 8 ∆A= -2.5 ∆B= -3 ∆A= -8 ∆B= -9

+

Ph

Carb. acids >> Phenol >> Amines > Water > Ketones

∆A= 7.5 ∆A= 2 ∆B= -2 ∆A= -3 ∆B= -8 Mg O H _H + O C O R Mg O H Mg O H _H + O Mg O H _H + O Mg O H N H R R Mg O H H R R Mg O H H O H Mg O H H C O R R

Ph

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result in a change of the acid-base equilibrium, resulting in mutual repulsion forces or in reduction of the degree of ionization of the organic group in either type A or type B interactions, as follows:

• Type A (basic surface with an organic acid):

As these reactions happen either at pH higher than the metal surface iso-electric point or at a pH lower than the pKA(A) of the acid, strong hydrogen- or ionic bonds will be limited to:

pKA(A) < pH < IEPS

Hydrogen bonds between a steel substrate and a mono-carboxylic acid are therefore limited to the pH range 4.5 – 8.5.

• Type B (acidic surface with an organic base):

Strong hydrogen- or ionic bonds will be limited to: IEPS < pH < pKA(B)

suggesting that hydrogen bonds between carbonyl groups and a steel substrate are never stable when attacked by water.

2.3.3.2 Coordinating

bonds

Coordinating bonds are defined as bonds formed between metal cations and electronegative atoms containing pairs of unshared electrons in the outermost valence shells like oxygen, nitrogen, fluorine. In case one coordinating molecule is able to provide two bonds to one metal cation, a very stable, water insensitive coordination complex or chelate may be formed.

Water or solvent molecules coordinate to metal cations through their lone pair electrons. Ferrous iron, for example, having 6 outer electrons, is able to receive another 12 electrons, forming an octahedral complex with six water molecules. Stronger bases than water can replace these coordinating water molecules producing metal complexes, shielding the cation from the solvent.

{ }

-MOH2+ HXR H+ H₂O H₂O OH -+ + -MOH X-R -MO HXR+ or -MO HOH + XR -MOH+ ₂+ HXR MO HXRH + H₂O H

{ }

or –MOH₂XR + H2O OH --MOH+ ₂+ HXR MO HXRH + H₂O H or –MOH₂XR + H2O OH

--MO HOH + HOH XR -MOH+ ₂+ HXR MO HXRH + H₂O H

{ }

or –MOH₂XR + H2O OH --MOH+ ₂+ HXR MO HXRH + H₂O H or –MOH₂XR + H2O OH

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Figure 2.7 shows an example of the interactions between hydrated ferric ions and a model compound (catechol) representing tannin molecules that are commonly encountered in tea49_{. The tannins are able to penetrate the first} coordination layer (substituting water) and react directly with the metal ion. Insoluble complexes develop on the tea surface after left standing for a while.

Figure 2.7. The reactions between a model compound for tannins (catechol) and iron ions at pH 1 to 749_.

This concept is used quite often for water treatment, to avoid scale deposition at alkaline pH, and for the production of catalysts containing iron that are stable at high pH and in sulphide containing environments(73,74)_{. As discussed} above, the stability of the hydrogen- or ionic bonds depends on the pH of the aqueous solution it is in contact with. If the pH is outside of this range, for instance higher than the metal IEPS, disbondment occurs due to displacement by hydroxide.

Although these chelating molecules are meant to react with iron cations in solution at various pH’s, and are not meant for bonding organic coatings to steel, an understanding of their properties will add to understanding the behaviour of coatings in aqueous solutions. In Chapter 6 polymers that produce coordinating bonds to ferrous and ferric oxide will be tested.

At this point, the large difference between metal hydroxides and organic alcohol groups must be emphasized. As metals are usually far more electropositive than carbon, the electron density on the oxygen atom in the oxide surface group is considerably higher than on the oxygen of water or any aliphatic alcohol. Organic alcohols normally undergo addition reactions (to isocyanates, epoxies) and condensation reactions (esterification, etherification) forming C-O-C bonds that are quite stable to hydrolysis.

Although metal hydroxyl groups are known to react through condensation reactions, forming M-O-C bonds(65,66,67,68,69)_{(for instance forming metal} alkoxydes), the highly ionic bonds are prone to hydrolysis in water. Due to the large negative charge on the oxygen atom, protonation proceeds rapidly,

OH

+

Fe 3+_aq O O Fe H2O H2O + ½ Fe3+_aq Fe H2O H2O O O O O O O + 2 H+ OH OH2 OH₂ - 4 H+

(31)

PET are far more stable to hydrolysis than metal carboxylates. Section 6.3.1 will deal with the resistance of coordinating bonds to hydrolysis in more detail.

In Chapter 3, siloxane coatings will be applied to steel substrates, assuming that M-O-Si bonds are formed. Numerous articles describe the formation of Si-O-Si bonds, and the benefits that (organo) silanes have on wet adhesion between polymers and glass (fibers). For obvious reasons, many attempts have been made to form the same type of bonds on metal substrates, too. Formation of a M-O-Si bond (“oxane bond”), whether having covalent or ionic character, stable to hydrolysis, would be the Holy Grail of metal coating. Up to now, evidence of formation of oxane bonds remains confined to the literature on Secondary Ion Mass Spectroscopy (SIMS). During SIMS measurements, ions are formed like (Al-O-Si )+_{and (Fe-O-Si)}+_{. Chapter 5 will} go into some detail concerning the type of interaction between siloxane layers and steel.

2.3.3.3 Description of ATR and IRRAS

The mechanisms through which organic molecules interact with hydroxylated metal oxide surfaces can be studied very well using infrared techniques. These techniques will be used in Chapter 4 (ATR and IRRAS) and Chapters 5 and particularly 6 (IRRAS). Therefore, a short introduction is given here of both infrared techniques used.

Transmission IR spectroscopy is generally a very useful technique for the analysis of organic compounds. However, the sample then needs to be thin enough or strongly diluted in a solvent in order to obtain a good quality transmission spectrum. The ATR technique (Attenuated Total Reflectance) made it possible to measure the infrared spectrum of samples in any form just by bringing it into contact with an ATR-crystal. Figure 2.8 shows what may happen when light, traveling through a medium with high refractive index n1 (the ATR-crystal) reaches the interface with a medium of low refractive index n2 (the sample to be measured).

At the point of reflection of the incident light within the ATR-crystal an electronic field (evanescent field) penetrates the sample with an intensity that decreases exponentially with distance from the crystal surface.

This evanescent field interacts with the electronic dipoles in the sample, lowering the intensity of the reflected radiation at certain frequencies. This physical phenomenon allows us to produce an FTIR-spectrum of solid and liquid (non-diluted) materials by just placing them on the ATR-crystal.

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Figure 2.8. The light passing through/reflecting at media of different refractive index (n2 < n1) is shown as a function of the angle of incidence ϕ (defined as the angle with the interface normal). ϕc, the minimum angle at which there is no transmission of radiation, is called the critical angle. Transmission at ϕ = 0°, reflection and refraction at ϕ < ϕc, reflection at ϕ = ϕc, and total (internal) reflection at ϕ > ϕc.

In the early 1960’s, the idea arose that the FTIR- technique could be used to study extremely thin organic layers on the surface of metals or semiconductors, by using their polished surface as the reflecting element. This technique is generally referred to as Infra Red Reflection Absorption Spectrometry (IRRAS). External reflection spectra depend upon the optical constants of both the substrate and the surface film. This is shown schematically in Figure 2.9.

Figure 2.9. Drawing of the metal substrate-organic film system.

The incident radiation can be separated into two perpendicular directions of

Θ

d

incident beam

reflected beam

p

s

Polished metal

p

2.4 Corrosion mechanisms of polymer coated steel

Corrosion of steel is an electrochemical process. The most important anodic reaction is the dissolution of iron. Corrosion of steel at ambient temperatures can only take place in the presence of water, being the only medium able to transport iron ions away from the anodic site. The anodic reaction is coupled to the cathodic reactions, of which the reduction of oxygen is prevalent in the neutral pH region, and the reduction of hydrogen ions becomes dominant in the more acidic pH regions. Organic coatings on steel substrates are applied to protect steel by providing a barrier accompanied by sufficient adhesion to the steel surface. Coatings are efficient barriers to diffusion of ionic species. Charge carriers like electrons and ions need to be transported between anodic and cathodic sites, in order to maintain electrical neutrality. Electrons follow paths through the metal substrate, whereas ions are transported by water (Figure 2.10 and 2.11). An extra addition of ionic species enhances corrosion reactions, because by lowering the electrical resistance of the electrolyte, they offer the possibility to anodic and cathodic reactions to occur at a larger distance from each other. In a simple but very interesting experiment, it was shown that PET is an effective barrier to sulphuric acid solution(11)_{. A} non-adhering thin PET film was placed on top of a polished and well-cleaned carbon steel sample at room temperature. On the PET film a cylinder was mounted filled with diluted sulphuric acid solution at pH 1. The cylinder was closed to prevent oxygen to enter the electrolyte. Due to the transparency of the PET film, the polished steel surface could be checked visually without the need to open the cell. After six months the iron surface finally became wet, but even then, due to the anaerobic conditions, no corrosion occurred. This shows, that in the absence of oxygen, and in the absence of water dissolvable ionic species on the steel surface, corrosion does not occur. Hence, the PET

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film effectively protected the iron against corrosion, implying that the H+_ions were not transported through the PET in significant amounts in six months’ time. Note that with this experiment, one is essentially measuring the transport of H+_{and HSO}_4-_{together, since diffusion processes do not transport} any net electric charge. As long as the coating acts as a perfect barrier to ions, perfect adhesion to a clean substrate is not of overriding importance. However, in normal life coatings are not perfect barriers as they usually show defects (to be defined as areas of lower polymer density) that can be penetrated by ions and oxygen molecules. Coatings made to protect packaging steel are strongly deformed during forming operations at sub-Tg temperatures, and at a very high forming speed. Forming operations were developed with the objective to produce metal packaging in the most economical way, and therefore it is very difficult to optimize these operations in such a way that cracks or local loss of adhesion of coatings can be avoided completely. Plastic flow of polymers at sub-Tg temperatures can be promoted by applying strong hydrostatic pressure, as for instance during the wall ironing process, whereas a low amount of biaxial stretching of the same coating may induce cracks or crazes. Electrochemical Impedance Spectroscopy (EIS) measurements and Open Circuit Potential (OCP) measurements on Draw Redraw and on draw and wall-ironed cans immersed in salt solutions show the onset of iron corrosion due to flaws in the coating in the can wall area usually within minutes. Sterilization heat treatment speeds up penetration by water and especially by ions tremendously, as this heat treatment happens at a temperature about 60°C above Tg of the wet and plasticized polymer.

Figure 2.10. The mechanism of the prevalent corrosion reaction at neutral pH on steel at the end of the first phase (anodic and cathodic reaction both still taking place in the defect zone).

During the onset of the corrosion reaction both the anodic reaction and the

Steel

Polymer

coating

Polymer

coating

Polymer

coating

Polymer

coating

oxide

O

₂

_{Electrolyte and O}

2

Ferrous oxide

Steel

FeOH

+

(35)

will be part of the corrosion reaction mechanism only if the rate of diffusion through the polymer coating or directly toward the defect zone is sufficiently high.

Leng(97)_{points out that, during the one-electron redox reactions, many} short-lived intermediates must be formed at the interface which might be even more destructive to the polymer coating than OH-:

O2 + e- O2 - . O2 - . + H2O O2H . + OH -O2H . + e -- O2H -O2H - + H2O H2O2 + OH -H2O2 + e- OH . + OH - and OH . + e - OH

-Anyway, the final product of the oxygen reduction reaction is hydroxide, and the first product of the anodic reaction is hydrated Fe 2+_{produced by:}

Fe + H2O Fe(OH) + + H + + 2e

-Depending on local circumstances like pH, the dimensions of the defect and the solubility products of the salts, a mixture of ferric and ferrous oxide and -hydroxide deposits may form anywhere on the walls of the defect zone: Fe(OH) +_{+ H}₂O FeOOH + 2H +_{+ e}-_{(anodic reaction)}

3FeOOH + e - Fe₃_O₄_{+ H}₂_{O + OH}-_{(cathodic reaction)} and

Fe(OH) +_{+ H}₂O Fe(OH)₂_{+ H}+

causing the formation of an ill defined and porous rust layer in the defect zone, through which diffusion of electrolyte and oxygen remains possible. However, diffusion of oxygen through this layer leads to scavenging of the oxygen molecules locally by ferrous ions. The concentration of ferric ions as compared to the ferrous form in the oxide increases, and no oxygen reaches the bare metal of the defect region anymore. Since the anodic reactions underneath the porous oxide layer lead to local acidification by producing H+ only, the presence of negative ions (like chloride) in the electrolyte would strongly support the corrosion reaction by migrating through the defective

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rust layer to neutralize the electric charge generated. The accumulation of ions in the defect area leads to osmosis of water, resulting in blister growth. The cathodic reaction (oxygen reduction by the electrons generated by the anodic reaction) takes place close to the defect zone underneath the polymer coating through which some oxygen diffusion is still possible. In this situation the anodic area is coupled to a ring shaped and small cathodic area because of the high ionic resistance of the coating and the limited lateral progress of delamination. This means that at the edges of the defect large quantities of OH-_{are formed within a relatively confined space. This leads to very high} local pH values, causing hydrolysis of the bonds between the coating and the metal substrate. This can be observed by introducing phenolphtalein pigments in the coating that turn pink at high pH(90)_.

Figure 2.11. Mechanism of the prevalent corrosion reaction at neutral pH on steel during the second phase (formation of an oxygen-depleted zone, formation of a galvanic element and cathodic delamination of the polymer layer).

Scavenging of oxygen molecules cannot take place in case of aluminium or chromium (that both form oxides with a cation valence of III only).

The hydroxide ions being formed in the polymer-steel interphasial region remain adsorbed there. The corrosion reaction continues if enough water is present in the coating matrix in the interphase region to remove the hydroxide ions from the surface. The presence of cations (like Fe(OH)+_and Na+_{) in the electrolyte will strongly support the corrosion reaction by} migrating toward the cathodic area to neutralize the negative electric charge of hydroxide evolving at the cathodic sites. Without the ability of cationic species to migrate to the cathodic site the oxygen reduction reaction would

intact

zone

intact

zone

FeOH

+

PET

FexOy FexOy

cathode

intact

FeOH

+

PET

FexOy FexOy

cathode

oxide

O

₂

electrolyte

O

₂

O

₂

Steel

e

-

(37)

-through the water swollen polymer coating, is accelerated by the potential difference between the active steel substrate (the defect) and the cathode region where oxygen reduction takes place. The process of oxygen reduction itself maintains this potential difference(92)_{. The diffusion of cations from the} defect site to the cathode close to the defect site but underneath the polymer coating determines the rate of the corrosion reaction(90)_.

The electrons, generated by the anodic reaction, must get through the oxide layer to reduce the oxygen. Oxygen reduction takes place inside the oxide scale and not necessarily on the metal surface: it is Fe(II) that is being oxidized, not metallic iron(84)_.

As the oxide layer of iron is not stoichiometric, and the iron ions can “flip” their valence between II and III, defect sites exist that make electron migration through the oxide and transfer to oxygen quite easy(84,88)_{. Therefore, reduction} of oxygen and the liberation of strongly nucleophilic hydroxide ions and short lived radicals is possible even on the passivated oxide surface of steel, which on the one hand stabilizes the oxide on the metal and inhibits any anodic metal dissolution underneath the polymer coating, but on the other hand induces destruction of chemical bonds in the polymer(97)_{and hydrolysis of} metal to polymer bonds, inflicting delamination (see Figure 4.5 of Chapter 4). The delamination rate can be enhanced by cathodic polarization(95)_. Delamination of the polymer coating and change in local potential of the steel substrate is detected very well by the Kelvin-probe set-up by the group of Stratmann(91,92,93)_.

Protection of the steel surface against corrosion at neutral pH regions is either possible by reduction of the density of donor states in the oxide layer by oxidizing Fe(II) to Fe(III) states and keeping them at that valence, or by application of a thicker or better barrier coating that hinders diffusion of oxygen, electrons and cations(80)_{. Various research groups have applied} silicon, silica or silicide coatings onto steel, with or without prior removal of the iron oxide(78)_{, combinations of silicate layers with siloxane monolayers}(79)_, phosphate layers (sealed by chromate)(85)_{or siloxane layers}(86,87,108-119)_{, and} succeeded in curbing the corrosion reaction.

All these pretreatments act by inhibiting the oxygen reduction reaction. By doing this, the potential difference between the anodic sites and the cathodic sites becomes smaller, resulting in a lower corrosion rate and less delamination(92)_.

The polymer layer inhibits the oxygen reduction reaction not by being a barrier to oxygen diffusion, but by being a barrier to migration of cations. Strongly adhering, crystalline, hydrophobic, high Tg polymers inhibit the formation of water pockets that permit ion diffusion.

Durability of polymer coated steel in diluted acetic acid environment