Deterioration of materials in marine environment

ARCHIEF

ADVISORY COMMITTEE FOR YACHT

RESEARCH

UNIVERSITY

Of

SOUTHAMPT

ON

department of

aeronautics

and astronautics

S B.Lit. Report No 30 DETERIORATION OF MATERIALS IN A MARINE ENVIRONMENT

Lab. v.

ScheepolvAlnde

Technische Hop.schoot

Delft

This report contains the papers presented at a one-day Symposium on

"Deterioration of Materials in a Marine Environment" at the University of

Southampton on 3rd April, 1970.

The Symposium was jointly sponsored by the Advisory Committee for Yacht

Research, University of Southampton and the Yacht Brokers, Designers and

Surveyors Association,

The Corrosion of Metals in Sea Water

Corrosion Prevention - Hovercraft Practice and Experience

Degradation of Reinforced Plastics

The Degradation of Wood by Metal Fastenings and Fittings

Prevention of Degradation of Wood

by Fungi

A contribution from Mr. P.E. White (G.K.N.

Editor; Dr. A. Millward

The papers presented in this report should not necessarily be taken

as expressing the opinions of either the Advisory Committee for Yacht Research

or the University of Southampton.

Mr. B.N. Hall

H.M. Dockyard, Portsmouth)

Mr, D.G. Reavey

(British Hovercraft Corporation) Dr. R. Dukes

(Admiralty Materials Laboratory)

Mr. L.C. Pinion

(Forest Products Research Laboratory)

Mr, J.G. Savory

(Forest Products Research Laboratory)

THE CORROSION OF METALS IN SEAWATER

B.N. Hall

Metallic corrosion has been defined as the reaction of a metal with its

environment. However, even with one particular environment several factors

may influence the corrosion process and can result in different types of

attack. It is intended to _discuss various forms of marine corrosion and,

where possible, to suggest methods of.avoiding or reducing attack.

Seawater corrosion is principally an electrolytic process and can be

illustrated by considering a simple cell consisting of an anode and a cathode

immersed in an electrolyte seawater)- If the anode is conne,-:ted to the

cathode via an ammeter the current flowing between the two can be observed.

At the anode, ions will dissolve into the electrolyte, i,e. the anode will

CORRODE:-M 1,1+4 (in solution) 2 electrons (Where M is the anode metal).

At the cathode, the 2 electrons will reduce oxygen in the

seawater:-2 electrons H2O i02 -> 20H₂ (in solution),

In practice, the anodic and cathodic areas can exist on one material due

to differences in alloy structure or availability of oxygen on the surface,

this will be discussed later but for the present let us consider two separate

electrodes.

The degree of corrosion caused by bi-metallic contact will be chiefly

dependent upon the relative difference in electrochemical potential. It will

be convenient to consider the galvanic series which places metals in an order

relative to one another. A study of the series, given in the Appendix, will

indicate the tendency of different materials to galvanic action. The closer

that materials stand to each other, the lower the degree of galvanic action

between them. _{It is emphasised that this guide applies only to materials in}

seawater and at ambient temperatures. Differences in the surface areas of the

two metals can infLuence_the. degree of attack; a large cathode area/anode area ratio will. increase current density causing greater severity of attack and visa versa.

Types of Marine Corrosion

Most alloys are not homogeneous .in micro structure but contain phases of

differing composition and electro potential. 60140 Brass, for example,

contains an 'a' phase which is cathodic to the zinc rich. 'B' phase and

corrosion of the B phase can occur. Even an 'all a' 70130 brass can suffer

attack due to the removal of the zinc atoms. This attack is usually referred

to as 'dezincification' and in the case of 70130 brass it can be inhibited by

the addition of a small amount of arsenic. Similar attack can occur on other

copper-base alloys, de-aluminification of aluminium bronze for example.

The result in both cases is a relatively weak copper residue having similar

dimensions to the original component, this makes identification difficult until

failure has occurred.

Cast irons in some environments can suffer a form of corrosion termed

graphitization which results. in attack of the ferritic matrix due to contact

with the cathodic graphite flakes, Again, the appearance of the component

might not suggest serious deterioration.

Crevice corrosion can occur when seawater has access to lapped surfaces

but where the rate of replenishment is low. An oxygen differential cell may

be set up the low oxygen area within the crevice being anodic to the outside

surface which is exposed to the normal amount of oxygen. Any damage to the

protective oxide film within the crevice will not be easily repaired due to

the low availability of ixygen and as the cathodic region outside is usually

much larger in area, attack can be rapid. Stainless steels are particularly

susceptible to crevice attack and are not recommended for underwater fittings.

The stainless steels which contain molybdenum as an alloying element appear

to offer much greater resistance to attack but they are not completely

immune,

Erosion-corrosion. mechanisms are a chief cause of trouble in Naval ships

but are unlikely to concern small craft to anything like the same extent.

Failure of water systems can occur due to impingement attack when fast flowing

or turbulent water breaks down the surface oxide film on the metal surface.

The exposed metal is continually oxidesed and removed. Entrapped air in the

water supply greatly accelerates attack. Impingement attack can be minimised

at the design stage by considerations aimed at reducing turbulence; in service

the risk of failure can be reduced by keeping water speeds in pipe and condenser

systems to within recommended limits. Selection of appropriate alloys can

avoid possible failure, for example, aluminium brass has superior impingement

resistance to Naval brass or Admiralty brass but is inferior to 90/10 and 70/30

cupro nickel. Cupro-nickels offer excellent resistance to attack provided the

specified small iron additions are made to the alloys. The other form of

erosion attack, cavitation, is mainly mechanical damage caused by the collapse

of cavities in the water supply at the metal/water interface. Propeller blades

are typical components which sometimes suffer from cavitation.

Stress-corrosion is attributed to the combined influence of corrosion

--and stress. The stress_may be external such as .a tensile stress, or internal

due to cold working in manufacture. The actual mechanism varies with materials

and even with slight .variations in environment but examples concerning

aluminium alloys may ...he relevant...to the interests represented at the symposium.

The use of analuminium 5% magnesium alloy for. riveting aluminium alloy

superstructures lead to numerous failures due to preferential attack of the

anodic S phase which is precipitated in grain boundaries:- Stress-corrosion

failure resulted in. the heads of rivets falling off. _{Replacement by an}

alloy containing 31% Mg., which is not susceptible to S precipitation,

eliminated trouble,

Corrosion fatigue failures occur when a fluctuating or alternating stress

causes a fracture to propagate from a corroded area. The corrosion, which may

for example be a pit on a shaft, acts as a stress-raiser. Such failures are

difficult to anticipate and fortunately do not occur too frequently.

Preventative Measures

The first considerations should be made at the-design stage; water systems

can be planned to reduce undue turbulence and precautions taken at bimetallic

joints to avoid galvanic action_by the use of suitable jointing compounds:

Consideration of suitable materials should take into account not only the

requirements of a particular component but the effects upon adjacent materials.

Indiscriminate use of titanium could result in attack on less noble alloys and

when replacing a copper-base alloy. with other-material marine fouling may

occur. The question of weldability is important when there is any likelihood

of weld repairs being necessary- New materials are of course worth consideration

provided their behaviour has been fully evaluated and the improved properties

-4-are necessary. In the field of copper-base alloy the recent introduction of

aluminium silicon bronze offers a medium strength alloy in both wrought and

cast forms which is attractive for marine use as it possesses good corrosion

resistance and weldability. The alloyed cupro-nickels now available offer

high strength and corrosion resistance. It is unlikely that the TO/30 type

containing Nb and Si would be of much interest to the small boat builder but

the copper alloy containing a lower proportion of nickel with manganese, iron

and aluminium additions could be considered as an alternative for such items

as propeller shafts on small craft, Hiduron 191 is an example of this

material in wrought form which would be compatable with brass or aluminium

bronze propeller materials. Reservations concerning the use of stainless

steels in seawater have already been expressed but new alloys now becoming

available such as Paralloy DPH and Ferralium, are thought to have reduced

susceptibility to crevice attack.

Protective measures after construction usually include a paint system

which has a decorative as well as a protective function, The degree of

protection will depend upon the good application of an appropriate system,

Cathodic protection may be considered for some craft and a sacrificial system

employing anodes which are replaced at intervals would probably be more

appropriate for small craft than an impressed current system, Recent

developments have been made in the field of aluminium anodes which, theoretically,

have cost and weight advantages. In practice the performance of some aluminium

anodes can be variable, zinc anodes however have been found to give reasonably

consistent performance. The installation of a cathodic protection system and

consideration of side effects such as the influence of alkali build up upon a

paint system calls for some knowledgable advice.

-5-APPENDIX

GALVANIC SERIES IN SEAWATER

CORRODED END (ANODIC OR LESS NOBLE)

Magnesium

Magnesium alloys

Zinc and Galvanised Iron

Aluminium alloys

Cadmium plate

Mild Steel, Wrought and Cast Iron

Lead-Tin solders Admiralty Brass H.T. Brass Copper Naval Brass Aluminium Bronze (BS 2032)

901515

Copper Nickel Iron

Gun metal

90110

Cupro-nickel

70130 Cupro-nickel NiCkel

Stianless Steel (EN

58 j) -

Active

Titanium (commercial purity) Silver

Monel

Stainless Steel (EN

58 j) -

Passive

Graphite

PROTECTED END (CATHODE OR MORE NOBLE)

-6

^

-CORROSION PREVENTION - HOVERCRAFT PRACTICE AND EXPERIENCE

D.G. Reavey

Introduction

Since all common engineering materials deteriorate under adverse

conditions provision must be made either by protective schemes or by ensuring

that catastrophic loss in properties will not occur within the economic life

expectation of the component. The hovercraft as an air riding vehicle is

weight dependent dictating the need for light-weight structures which,

because of the aggressive marine operating environment, give rise to material

problems peculiar to it. In this paper the basic approach to the problem,

work on the evaluation of protective finishes, and service experience of

these will be discussed.

Design

The choice of structural materials is a compromise between strength,

environmental compatibility, weight and cost, Principal among those used

are aluminium alloys, corrodible and non-corrodible steel, PVC foams, wood

and glass reinforced plastics. With the exception of the latter direct

allowance for deterioration is not made the emphasis being on avoidance

effected

by:-Correct material selection

Attention to detail avoiding features which cannot be adequateiy protected

Avoidance of crevice corrosion from water entrapment in joints

Specification of adequate protective finishing standards.

Each of these parameters must be considered in detail for the success

of a corrosion preventative scheme is dependent on each of them and can only

be as good as the designer allows. Efficient protection can only be built in

on the drawing board. It has been our experience that schemes evolved at a

later stage of manufacture usually cost more to supply and were rarely so

effective.

The Selection of Basic Structural Materials

The limitations imposed by operating environment dictated that corrosion

prone alloys such as magnesium and the ultra strong aluminium alloys should

not be used. However, at the other end of the scale corrosion resistant

materials such as stainless steels and titanium were not used extensively

because of cost or weight considerations.

In the basic aluminium alloy structures the wrought alloys were

principally N8, 1130, HC15 and T415. The marine corrosion performance of the

first three is good but the latter is more susceptable and its application

has been limited to internal structure where the environment is less arduous.

Construction of a craft in one alloy to avoid the problems of galvanic

corrosion arising from dissimilar metals was desirable, but clearly impacticable.

In material selection the designer was obliged to consider electrode potentials

given in Table I to avoid couples in excess of 0.25 volt. At first sight this

may appear impracticable but there is a simple solution. Taking the case of

aluminium fastened with a stainless steel bolt it will be found that the

potential difference between them is (-0.75) - (-0.20) = 0.55 volt. By

cadmium plating the stainless steel (-0.80 volt) the couple is reduced to

acceptable proportions.

-The risk of galvanic corrosion is further reduced by wet assembly

techniques.

Protection of Joints

It is general engineering experience that a great many corrosion

failures originate from joints, either from the interfay or around fasteners,

Marine hovercraft must of course be watertight and there is thus a two fold

reason for ensuring that water cannot penetrate into the joints. In some

joints this is achieved by structural adhesive bonding while mechanically

closed joints are wet assembled using polysulphide or polyurethane rubbers,

having corrosion inhibitive properties, in the interfays, It is important

that such joints are closed before the compound cures to ensure joint sealing

and also to achieve a degree of self-filleting by 'spew out Filleting at

the edges of joints is important in that water entrapment is minimised and a

smoother contour obtained for subsequent painting, As corrosion originating

from fasteners can be troublesome, particularly with rivets of the 'pop' type,

these are wet assembled before closure.

Application of priming paints at the detail stage of manufacture, that

is prior to assembly, gives additional protection.

Finishes

Considering paints only it was our experience that while reasonable

degrees of corrosion resistance could be effected by them alone on certain

aluminium alloys they did not generally confer adequate protection on steels

For these electro-deposited cadmium in conjunction with paint gave a

reasonable degree of protection but the best results were obtained with

sprayed aluminium as the basis for the paint0 For this reason we specify

-9-cadmium only for close tolerance parts.

In selecting paint schemes the: following basic properties were

examined:-Flexibility

Impact resistance

Adhesion

Colour and gloss retention

Corrosion resistance relative to paint weight/thickness

Cost of materials and application.

Figures 1, 2 and 3 illustrate the variation of flexibility, impact and

corrosion resistance respectively of various schemes. The simulated joint

specimen shown in Figure 3 is preferred for corrosion testing of protective

finishes because it has been_our experience that breakdown in service occurs

more rapidly at joints than on plain surfaces.

Earlier aircraft and general experience served to reduce the number of

contending schemes but

a

major problem, whether to paint as a ship or

aircraft, had to be resolved. An obvious disadvantage of many marine

finishes was their weight while the durability of aircraft finishes for this

application was not established. By experience on development craft and

laboratory evaluation the scheme which we have specified since

1965

was

evolved. This

comprised-A phenolic modified etch primer

A strontium chromate pigmented epoxy primer

An epoxy/polyamide finish.

The particular etch primer was preferred for its adhesion and water

resistance, the epoxy primer because the strontium chromate pigment gave the

-

10-le)

highest degree of corrosion inhibition. Standard marine epoxy primers

pigmented with barium or zinc chramates at film weights of 1-2 oz/sq.yd. did

not give comparable protection when tested on a simulated joint specimen.

Work by a number of investigators attributes this to the manner in which

the chromate behaves in the presence of moisture showing that the degree

of protection a chromate pigmented primer confers on aluminium alloys may be

related to chromate leach rates, In Figure 4 the relative leach rates of

strontium, barium and a mixture of strontium and barium chromate pigmented

epoxy primers is shown. Our experience, lased on half tide exposure and salt

spray corrosion tests, confirmed that strontium chromate pigmented primers

were superior to those containing barium chromate. However, we also found

that some trontium chromate primers with high leach rates did not confer

lasting protection because available chramates became exhausted. The best

results were obtained from a strontium chromate primer compounded to give

a moderate prolonged leach. The epoxy finish chosen was preferred because

it had good flexibility and came from the same manufacturer as the primers,

This scheme has given good service in terms of corrosion resistance

but the shortcoming of epoxy finishes has been their relatively poor colour

and gloss retention when exposed to light. Internally the epoxy has been

good and, apart from mechanical damage, internal painting has been unnecessary.

However, craft must generally be repainted externally at 1-2 year intervals.

Polyurethanes were investigated at an early stage and although superior in

terms of colour and gloss retention were rejected because of their poor

flexibility and impact resistance. It was found that on impact, because of

their high cohesive strength polyurethane paints chipped away taking the

primers with them to expose the base metal and there was a tendency to crack

-along the lines of flexing joints. There were also problems in abrading to

achieve good intercoat adhesion on repainting.

Recently more Ilexible_polyurethanes have been developed. Though a

little inferior to the harder polyurethanes in terms of colour and gloss

retention they are much superior to the epoxies and intercoat adhesion of

finish-to finish is good. Two seasons service experience indicates craft

repainting frequencieson predominantly white schemes, should be at least

three years.

Conclusion

Our experience over the last decade has shown that by attention to detail

from the design stage onwards amphibious hovercraft, can, and have been,

effectively protected against corrosion.

Laboratory and service evaluation of a range of paint finishes revealed

considerable variations in the degree of protection conferred by them on

hovercraft structures and highlighted a number of shortcomings. From this

work it was concluded, and subsequently confirmed by usage, that a satisfactory

scheme in terms of corrosion resistance

comprised:-A phenolic modified etch primer

A strontium,chromate pigmented epoxy primer

An epoxyholyamide finish.

Recently more flexible polyurethane finishes have been developed with

better gloss and colour retention than the epoxies and are now replacing them

for external applications.

12

-a)

qi)

-25°C.

TABLE I

E.M.F. between a Calomel electrode and various metals in sea water at

13

-E.M,F, (volts)

Magnesium and its alloys -1.60

Zinc die casting alloy -1.10

Zinc plating on steel -1.10

Zinc plating on steel, chraaate passivated -1.05

Galvanised iron -1.05

Tin zinc (80:20) plating on steel -1.05

Cadmium-zinc solder -1.05

Wrought aluminium alloys (clad) -0.90

Cadmium plating on steel -0.80

Aluminium alloy castings -0.75

Wrought aluminium -0.75

Steel and gray cast iron -0.70

Duralumin type alloys (unclad) -0.60

Lead -0.55

Lead-siver solder (2i% Ag) -0.50

Tin-lead solder -0.50

Tinned steel -0.50

Chromoum plating (0.0005") on steel -0.50

Stainless steel 12% Cr -0.45

Tin plating on steel

Cr. plating (0.00003") on Ni plated steel

Chromium -0.45

Stainless steel 18% Cr 2% Ni -0.35

Copper (Brasses and bronzes) -0.30

Nickel - copper alloys -0.25

Stainless steel 18% Cr 8% Ni -0.20

Silver Solder -0.20

TABLE I (Contd.)

14

-E-M.F, (volts)

Monel -0.15

Nickel plating on steel -0.15

Titanium -0.15

Silver and silver plating on copper 0

Rhodium plating on Ag plated Cu +0.05

Carbon +0.10

Platinium +0.15

-x

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DEGRADATION OF REINFORCED PLASTICS

R, Dukes

Introduction

The problem of environmental degradation of reinforced plastics is

becoming of greater importance as these materials are used in increasingly

arduous applications and in more efficient structures Water and applied

load are the two major sources of degradation in a marine environment although

weathering and chemical attack may also be of importance Biological attack

of glass reinforced plastics does not seem to have occurred although this

excludes marine fouling which is a problem although it does not directly

degrade the composite material.

(a) Water Degradation

The resins used with glass reinforcements have the ability to absorb

water, The actual amount varies from type to type and between individual

resins within a given type so that although the water absorption curves in Fig,1

are typical they should not be taken to apply to any particular resin,

The result of this water absorption and any finding its way along

fibres and through crazing is to degrade the material strength and the effect

on mat composites is illustrated in Fig.2, Again the results are only typical,

2. (b) Load Degradation

Considerable damage and even outright failure can be caused by long

term and cyclic loading of g.r.p, and both these effects can be accelerated

by the presence of water. Typically the stress to cause failure of a mat

15

laminate in one million cycles may be only half of its short term ultimate

tensile strength, while a tensile stress of 70% of the short term tensile

stress could cause delayed failure after a period of several months.

3. Acknowledgment

The author wishes to thank the Director, Admiralty Materials Laboratory, for permission to give this paper.

It should be made clear that the views expressed are those of the

author and do not necessarily represent Navy Department opinion,

-2'

200

400

600

Time (days)

Fig. 1

Typica:

water absorbtion ,curves

for

polyester resins.

--

finis h.

'1A

Finish 'El'

Time (days)

Figk 2

Loss of strength

of

mat laminates.

lOrt hoph t halit

polyester

Ilso ph thalic

polyester

Bis phenol 'A '

polyester

lexwal

strength

eaS

30

Li

₂₀

H-t;

1 0 10 (.7)' 0

Ten site

strength

600

200. 400

THE DEGRADATION OF WOOD BY METAL FASTENINGS AND FITTINGS

L.C, Pinion

As well as the hazards of biological decay, timber used in boat building

is subject to the effects of chemical decay associated with the corrosion of

metallic fastenings. The title has been deliberately chosen to emphasise that

in wooden construction the troubles are not just those of corrosion of the

fastenings, but also the destructive secondary effects on the wood caused by

the products of the corrosion processes (Fig 1), For centuries the world has

lived with these troubles commonly referred to as "nail sickness". In contrast

to biological decay the chemical decay associated with corrosion is confined to

the wood adjacent to corroded fastenings and it is regrettable that in repair

work so much wood has to be replaced because of a small percentage degrade in

vital spots.

Before methods can be devised to overcome these troubles it is necessary

that a better understanding is reached of the mechanisms operating in the

degradation

process, and recent work at FPRL has thrown some light in this area.

The general problem

will now

be considered in some detail followed by suggestions on possible preventative and remedial measures

The Basic Princi les of Electrochemical Corrosion

Corrosion is the result of an electrochemical process in which the

corrosive effect is proportional to the current which flows between areas of

potential difference- There are a number of causes of this electrochemical

effect such as dissimilar metals in contact, differences in concentration of

some chemical factor such as the electrolyte or oxygen availability, stress etc,

-In all cases areas of different polarity are produced and corrosion only

proceeds when the electrical circuit is completed by an electrolyte bridge

e.g, sea water, and a conductor between the anodic and cathodic areas. Fig.2

shows a simple diagram of electrochemical corrosion

Anodic and Cathodic Processes

Taking the case of iron (Fe) as an anode, and common salt (NaC1)3 as the

electrolyte, the following electrochemical effects take place (Fig.3).

Electrons (negatively charged particles) flow from the anode to the

cathodic surface where electrochemical action with water and the positive

ions (Na+) of the electrolyte produce hydrogen (H2) and caustic soda (NaOH)

At the anodic surface, iron metal, due to the loss of electrons, passes into

solution as ferrous ions (Fe) forming a soluble metallic salt _{(FeC12) with}

the negative ions of the electrolyte (C1).

Corrosion always occurs at anodes, and the cathodic areas always become

less acid and often highly alkaline.

In wooden vessels where salt water is absorbed into the wood to an

appreciable depth, the wood becomes a matrix for holding the electrolyte and

forming a bridge between anodic and cathodic areas.

The permeability of the wood is thus a major factor, for if the electrolyte

can be excluded, little corrosion can occur. A much longer hull life can be

expected if impermeable timbers are chosen, and impermeable surface coatings

are maintained.

Cathodic areas can often be easily identified in wooden vessels as much

of the alkali is retained in the surrounding wood and can be detected by test

papers or phenolphthalein solution. Likewise, in the anodic areas the metallic

-corrosion products are retained in the wood often causing heavy staining, It

is the retention of corrosion products and their destructive effects within the

wood which complicates the effects of corrosion in wooden hulls, presenting

problems which do not occur in all-metal construction.

Degradation of Wood in Cathodic Areas

The effect of alkali produced at cathodic surfaces is to attack the

surrounding wood which becomes soft and spongy,- impairing the holding power of

fastenings. No wood is immune from attack by caustic alkali, although denser

woods give better service simply because more wood substance is present.

Softwoods, because of their composition, have a greater resistance to alkali

than hardwoods which contain a much higher proportion of hmmicelluloses and

other components which are dissolved by alkali For this reason alkaline

attack on hardwoods is usually accompanied by a greater shrinkage and

distortion. Impermeable woods, by resisting the penetration of salt water, are

not so prone to the setting up of corrosion processes as permeable timbers,

However should corrosion arise in fastenings in impermeable timbers the alkali

produced cannot diffuse so easily, leading to high alkali concentrations and

localised chemical decay around the fastenings. A permeable wood species may

not suffer greatly in a cathodic area as the alkali produced can be diluted

by dispersion or even washed away. The presence of a small amount of alkali

promotes water absorption, and by rendering extractive components more soluble

increases the peLmeability of the wood. Increasing concentrations of alkali

progressively attack the more resistant hemicelluloses, lignin, and cellulose

components of wood.

19

-Degradation of Wood in Anodic Areas

Consideration wi:1 now be given to the effect on the wood of metallic

salts produced by the corrosion processes of the anodic area. The salts of

some metals, notably copper and zinc, are used in some preservative treatments,

and there is no evidence that they cause degrade in the timber. However there

is evidence that iron compounds degrade wood, causing softening and impairing

the mechanical strength to a considerable degree. Recent French work (Ref,3)

on the degradation of railway sleepers considers iron corrosion products as a

major factor governing the useful life of sleepers. _{Details will be shown}

later of the oak frames of a fishing vessel which became considerably

softened due to the action of iron corrosion products on the wood. It was

possible to drive a six inch nail with an ordinary carpenter's hanuner into this softened timber

As the corrosion products of many metals are coloured and often cause

staining in the wood, the occurrence of stains is often a good indication of

the presence of anodic surfaces and corrosive activity. In tannin-containing

woods such as oak and the majority of tropical hardwoods iron salts produced

at any anodic surface will diffuse along the grain of the wood interacting with

the tannin to produce the familiar blue-black iron stains,

Where the soluble corrosion products from the anodic and cathcdic

surfaces meet in diffusing through-the wood, the alkali causes precipitation

of the metallic corrosion product, causing rust deposits in the case of iron

which vary in colour according to oxygen availability

The Problem of Chemical Decay

The prevention of chemical attack on the wood can best be accomplished

by controlling the primary cause, that is the corrosion of the metal of the

-fastenings, In simple terms, the solution to the one problem is the solution

to the other.

Galvanic Corrosion (Bimetallic Action)

Some of the reasons for corrosion in wooden vessels are well known;

for example, dissimilar metals in contact in saline conditions are likely

to produce areas of different potential leading to severe galvanic corrosion

with chemical attack on the wood- These troubles can be considerably reduced

by isolating the metals concerned, or by using one kind of metal or alloy,

These are all matters of good design,

Corrosion in Isolated Fastenings anden Availability

Many of the reasons for corrosion in wooden boats are much more obscure.

Quite often cathodic areas can be identified where there is no direct contact

between dissimilar metals, and even in cases of isolated fastenings involving

one metal, alkaline spots can be observed,

Recent work at FPRL has led to a better understanding of the troubles

which arise in isolated fastenings, The principles which are much the same

as in the crevice or pit corrosion of ferrous metals, have not been generally

recognised or identified with the underlying troubles of metal fastenings in

wooden construction,

In the study of the deterioration of galvanised iron fastenings, amongst

the experiments set up were a few simple ones involving isolated rods of mild

steel and zinc in various timbers soaked in both fresh and salt water, Iron

in all cases developed alkali at the exposed ends indicating the formation of

a cathodic surface. Examples of alkali produced with iron in beech, oak and

mahogany soaked in sea water are shown in Fig,40 This condition was soon

-established with beech saturated in sea water, and surprisingly _{soon afterwards}

in samples saturated with fresh water, although the electrolyte available from

water and wood must be very limited in the latter case. _{Internal corrosion}

derived from anodic surfaces was made evident in many woods by extensive iron

staining which had travelled along the grain of the wood Although alkali

could be detected at the exposed ends of the zinc samples the amount was very

much less It is well known that the products of corrosion from zinc are much

less soluble than those of iron, producing greater electrical resistance and

diminished corrosion activity. Although zinc compounds are usually white or

colourless and do not normally form coloured complexes with organic materials

the distribution of mould growth provided some indirect evidence that

corrosion of the zinc rod had occurred within the wood. Mould grew on all

surfaces apart from the grain in the plane of the zinc rod, which suggests the

poisoning effect of zinc salts

The reason for the occurrence of corrosion cells in these cases is due

to differences in oxygen

availability

The experiment illustrated

in

Fig-5 demonstrates this point Two identical iron rods, one of which is

embedded in a piece of wood saturated with sea water, are placed in a glass

vessel also containing sea water, The iron rods which are connected with

leads through a microammeter register a small current, the unprotected iron

behaving as a cathode, This is to be expected as the unprotected iron has

better access to dissolved oxygen, If the concentration of dissolved oxygen

in the sea water is increased by blowing in air, the current flow shows a marked

increase, indicating a corresponding increase in corrosion activity, In most

cases of corrosion of iron in wet situations, the controlling factor is the

rate at which oxygen can reach the cathodic area, Most co

22

and alloys will develop corrosion cells if embedded in a wet matrix showing

differences in oxygen availability,

Consideration will now be given to a serious case of corrosion which

occurred in the galvanised iron fastenings of a fishing vessel having beech

planking below the water line, fastened to oak frames. The rest of the

planking was oak in which the fastenings were still satisfactory after over

twenty years of service. The case is illustrated in Figs.6, 7, 8 and 9.

The fastenings in the beech planking were severely corroded, in some cases

being completely consumed apart from the nail heads (Eig.7). Thick rust

deposits were common-place between the contact surfaces of planking and

frames (Fig.9). Damage which was of greater concern from the repair point of

view was the softening of the heavily iron-stained oak frames adjacent to the

beech planking, impairing the holding power of nails. Further examination of

boats having beech planking below the water line has established that such

construction is prone to corrosion of the galvanised iron planking fastenings.

In this case the interrelation of corrosion and chemical attack has gone full

circle in that a permeable timber has provided access of oxygenated sea water

to the fastenings bringing about corrosion, and the corrosion products in turn

have degraded the wood, particularly the impermeable wood of the frames.

The initial stage of corrosion was most likely the depletion of the zinc

coating within the oak, due to differences of oxygen availability The

remainder of the zinc would then be lost by galvanic action followed by attack

on the iron fastening due again to differences in oxygen availability.

It is noteworthy that the corrosion was most severe at the bows of the

vessel where the motion of the bow wave would tend to increase the dissolved

oxygen content of the sea water in contact with the hull.

-Choice of Timber Permeable and Im ermeable Woods

Most of the traditional timbers used in boat building such as oak, teak,

mahogany, pitch pine, and others held in high esteem such as iroko and afrormosia,

are all impermeable timbers. Difficulty in sea-water penetration in these

timbers creates difficulty in the formation of corrosion cells due to a scarcity

of electrolyte and oxygen supply. Permeable timbers allow easy penetration of

electrolyte i.e, sea water, and the rate of corrosion that follows is governed

mainly by oxygen accessibility

Wise Choice of Materials and Good Design

Although the reasons for corrosion of fastenings and nail sickness of

the wood may not have been clearly understood in the past in the boat building

industry, many of the practices of high class construction were most effective

in preventing these troubles- The choice of durable, impermeable timber and

the practice of counterboring planking to take fastenings, followed by stopping

and plugging with a cross-grain dowel effectivley excluded electrolyte and

oxygen from the metallic surfaces.

Fastenings of metals and alloys which are less prone to corrosion, make

a valuable contribution to overcoming these troubles, but these materials are

mostly very expensive,

Fastenin s and the Possibilit of Plastic Protective Coatings

Galvanised iron fastenings are often employed in the belief that

protection of the iron is ensured to give a reasonable life- This may be so

in fittings exposed to a marine atmosphere, but it is very questionable whether

the coating survives very long in wood wet with sea water unless the surfaces

are protected to isolate the metal surfaces from electrolyte and sources of

-24-oxygen. Fig. 10 shows an appreciable amount of alkali generated

in

4 few

hours from a galvanised iron nail in a beech block saturated with sea water.

Plating fastenings with another metal does nothing to break the electrolyte

circuit as it is still a conducting surface, and in many instances electrochemical

corrosion occurs. If an impervious nonf-conducting coating could be applied to

the surface of fastenings, there would be no electrolyte contact with the

metallic surfaces, and all forms of electrochemical attack would be prevented.

Loose plastic sleeves and washers have sometimes been used to isolate bolt-type

fastenings from wood. These cut down corrosion and chemical attack on the wood

but the electrolyte bridge has not been eliminated, and troubles are merely

reduced. Certain plastics such as poxides and polyurethanes adhere strongly

to metals giving impervious non-conducting films. A considerable interest

has

been shown especially in the United States (Refs04 and 5)

on

improving the holding

power of nails by using plastic coatings, but little attention seems to have

been shown in the uses of plastic coatings to prevent, the corrosion of

fastenings in permanent wooden structures. Further research and development

into impervious, nonconducting coatings for fastenings is needed,.

If

durable

coatings, which can Withstand driving can be developed, the corrosion problem

in wooden boats can be brought under better control, and the long life of high

class Wooden construction can be extended even further. If coatings of a very

hgih standard can be developed, concessions could possibly be made in accepting

permeable timbers, for construction provided matters, concerning biological decay

are attended to, Considerable saving could possibly be made in the cost of

timber and fastenings.

Commeuts_on Modern Materials .,and Fittin s

Many modern forms of wooden .construction such as hot, and cold moulded,

25

-hulls, and plywood, will suffer less from the troubles described, _{as there are}

far fewer joints where salt water penetration can arise, However, should sea

water penetration occur with access to a source of oxygen, corrosion troubles

and timber degrade will take place,

Concerning fittings, every attempt should be made to break the conducting

path between metallic surfaces and wood which is likely to become contaminated

with sea water. The back of metal fittings should be treated with

non-conducting impervious materials like bitumen or epoxide resins. It is

particularly important for deck fittings which should be set in bitumen or

mastics to exclude sea water from the joints. The danger in fittings with

metallic surfaces exposed to the air is they may serve as large cathodic areas,

causing severe corrosion in the fastenings attaching them to woodwork. If

impervious coatings could be applied to all metal surfaces to exclude both

sea water and oxygen the troubles of corrosion and chemical decay in wooden

construction would largely disappear.

-References

1. Bulletin 31, Prevention of Decay in Wooden Boats, Forest Products

Research Laboratory, RMSO.

Timber and Plywood in Boat Building. Electrochemical Corrosion and

Decay, Timber Research and Development Association,

J. SAVARD et L. CAUMARTIN Etude de la Degradation (rune Traverse

Bois et Forets des Tropiques, Sept/Oct 1969, 61-66,

R.S. KURTENACKER, Performance of Container Fasteners Subjected to

Static and Dynamic Withdrawal, US Forest Service Research Paper, Forest Products Laboratory, June 1965.

T.J. ALBERT and J,W, JOHNSON, Lateral Holding Capacity of Power-Driven

Fasteners, Forest Products Journal (Sept 1967), 17, 9, 59-67,

Acknowledgment

This paper is published by permission of the Ministry of Technology,

4.

ANODE

Metal corroded

Fig. 1 Chemical decay of wood due to electrochemical action. Note the

alkaline area around the fastening.

tt.ECTRON FLOW

CONDUCTOR

E

Part solutw9 Cathodic becomes or more CATHODE IRON ANODE 2e E.L.EcTRoNs. 2e SALT WATER-'

Fe** 'CI Na 'OH H H2

'Cr NaOH H*I

CATHODE

area

less acid FERROUS SODIUM HYDROGEN

Alkaline. CHLORIDE HYDROXIDE (Removed by (Corrosion (Alkali) atmospheric

product) oxidation)

Fig 2 Principles of electrochemical corrosion. Fig. 3 Electrochemical corrosion of iron in salt water.

JAA rt.i

-965 1/j '01 I, Epoxide resin sealing, I 1 Li II 111 U t I PA IIriE .41J _I DE 6,1 10 M;t_ABS 111 r . "

Fig,. 4

'Electrochemical

corrosion

of

iron rods in woods

Soaked in sea water.

Beech oak mahogany. Note

the iron staining

and the production of alkali

in

all

cases.

Microammeter

Mild steel

rods.

[14 Ii

Fig. 5

Corrosion

activity

due

to

differences

in

oxygen

availability.

,

' JP' g 1 EL .1 E Er nu e if N ' " "UE11 2 E ri E II 6 6 66 ELI 611 EU 6,IIIII1 11,6 EIP 61 11111 n El t'Ir5 Wood

OAK

OAK

No

serious

corrosion

after

20

years

Iron staining, softening of timber Rust deposit

Severe

corrosion

Anodic depletion of iron. Cathodic area Alkaline)

Permeability of Beech allows access

of oxygenated sea water

Fig. 6a

Oak planking on

oak frames.

Fig. 6b

Beech planking

on

Fig. 7 The

corrosion

of

galvanised iron

nails from beech

planking

on oak

frames.

Fig. 8

Corroded

galvanised

iron dumb bolts from

beech planking on

AC, *qn trol; 471 _3 4' r-O'r P AA--r., oo It COI vo , 4 nnrnnv.nir aL 1 1 itI 1[1 1111 '.716.11eMILL

Fig. 110

Electrochem leaf corrosion

of ,galvanised iron

nail in 'beech

soaked in sea water. Note the absence of alkali in

a

similar

nail

coated

with epoxide

resin,.

-r e

Fig. 9

Rust

deposit

between beech planking

PREVENTION OF DEGRADATION OF WOOD BY FUNGI

J,G. Savory

Wood is the oldest and most versatile of constructional materials but

during the present century great changes have taken place in availability of

timber species, sources of supply and modes of handling timber before use

Advances in timber technology have done much to replace the losses of

traditional craft knowledge consequent upon change but the net result is that

greater reliance must be placed on technical knowledge if wood is to be used

to best advantage

Attack of wood by biological agencies

One of the advantages of wood is that timber species of different

properties provide material suitable for a wide range of uses but the way in

which wood is attacked by a range of living organisms is a major disadvantage

Insect attack in boats is rare and can usually be related to failure to reject

unsuitable timber Marine borer attack is of greater importance especially

on the piling of wharfs and jetties but decay caused by fungi is by far the

greatest problem, There are two main types of fungal attack, that caused by

the wood-rotting Basidiomycetes, species of Coniophora and Poria being

commonest on boats, and slow superficial decay called "soft rot" which is

caused by microfungi. Soft rot is found primarily on submerged timber but

has also been noted on mahogany cabin tops following persistent leakage of

water into open joints.

Conditions leading to

Decay can only arise as a result of of the wood either in situ

28

by air-borne fungus spores or because of the use of wood already containing

infection, Fungus spores germinate only when atmospheric humidities are very

high and decay will not develop unless the wood moisture content is in excess

of 20 per cent.

Oxygen is essential for fungus growth- The mall amounts dissolved in

water are insufficient for growth of the Basidiomycetes so these fungi do not

attack waterlogged wood. In contrast, the soft-rot fungi can utilise

dissolved oxygen and so cause slow decay of submerged wood.,

Temperature governs rate of decay. It is slow below 10°C (50°F) and is

usually most rapid at warm summer temperatures around 27°C (80°F) _{It follows}

that decay is more rapid in the tropics than in temperate regions.

Though fungi do not grow at temperatures near freezing they are not killed

even by extremely low temperatures. On the other hand they are killed by high

temperatures, Steaming or kiln treatment sufficient to raise the wood

temperature to 65°C (150°F) will sterilise infected wood.

Natural decay resistance of timber

Sapwood is not resistant to decay so it should not be accepted for most

marine usages- Heartwood decay resistam.e varies with the position in the

tree. Wood near the pith is often less resistant: Resistance of different

timber species also varies widely. Teak is a classic example of a very

resistant timber. European oak is the most resistant of our native timbers.

American red oaks are less resistant and can be loosely grouped together with

African mahogany, Oregon pine and larch, Elms are less resistant still,

though probably rock elm is more resistant than white elm, and they are grouped

together with gaboon and the spruces as being non decay-resistant. The lowest

-decay resistance class of all includes timbers such as ash, beech and the

birches, More detailed information on decay resistance and other timber

properties is obtainable from the FPRL publications,

Variations in decay risk

Low decay risks in waterlogged timber and higher risks in tropi,a1

climates have already been noted, Risks differ also with the type of boat

and mode of usage, Small sailing dinghies which spend comparatively little

time in the water and can easily be stored under cover present few decay

hazards, as illustrated by the columon use of gaboon plywood in their

construction-gaboon being a timber which is non-resistant to decay,. Larger decked boats

are more liable to trouble, firstly because bigger dimensioned timbers are

more difficult to dry when wetted and, secondly, because provision of adequate

ventilation becomes more difficult.

In the individual boat, points of exposure of end grain, as at bolt holes

and joints, are a source of trouble because of the greater ease of penetration

of moisture Regions prone to wetting by leakage such as the beam shelf area

or regions in which ventilation is restricted also present greater decay risks_

In contrast, freely ventilated engine rooms. especially in working boats, are

comparatively free from decay,

Use of wood preservatives.

The value of preservative treatment is governed more by the extent to

which the preserving chemical penetrates the wood than by anything else. It

follows that, on occasion, it is desirable to choose a timber which can be

easily impregnated with preservative, for example treated red oak is accepted

by the United States Navy.

30

-Vacuum and vacuum-pressure treatments in special plant can ensure deep

penetration of permeable timbers. Treatments by commercial deluging or dipping

processes give more superficial protection but prolonged soaking can also lead

to deep penetration. _{Brush application of preservative is generally least}

satisfactory but even this can give useful protection if care is taken to

make copious applications at joints and faying surfaces.

Laminated members and plywood

Laminated construction provides members of greater strength which can

be precisely manufactured to requirements but in addition small laminates can

be more effectively treated with preservative than large members. However when

preservative treatments are carried out prior to assembly, care must be taken

to ensure compatibility between the preservative and the glues. This subject

is discussed at greater length in FPR Technical Note No 31.

Modern resin glues retain their strength under moist conditions but the

process of gluing does not confer decay resistance, hence decay resistance

both in plywood and in laminated construction has to be sought either by

choice of durable timbers or by preservative treatments It is to be regretted

that British Standard 1088 allows the inclusion of "small proDortions" of

sapwood, Thus, in situations where there is real risk of fungal attack, it is

advisable that preservative-treated plywood to BS 4079 should be used.

Reinforced plastic

Precautions against decay still need to be taken when wood is used in

combination with reinforced plastic materials Even if the wood is completely

encased, the physical protection given by the plastic is likely to break down

in time Decay risks then arise, for moisture which gains access to the wood

-through breaks in the plastic cannot readily dry out again,

Because of their resistance to abrasion, reinforced plastic sheathings

applied to one face of timber are superior to most other coverings in preventing

entry of water. However if extensive use is made of such coverings, for

example by sheathing the whole of the hull, then greater demands are made on

the internal ventilation for no moisture will be lost through the ,overing

Decay prevention

The design of a boat is inevitably a compromise between conflicting

requirements. In addition to providing a tight hull, design should minimise

risks of entry of water at all points. Rain water is especially dangerous

for most decay can be traced to the presence of fresh water. Duff's

emphasis on the need for careful design of the deck edging plank and bulwarks

to reduce the effects of leakage, if not to prevent it altogether, serves as

a good illustration of these points. Specification of edge sealing of plywood

is particularly important because, at the very least, entry of water will cause

discoloration under clear finishes.

Provision must be made for easy removal of moisture from within the boat.

Weep holes must be sited to allow free drainage into the lowest part of the

bilges whence water can be pumped away. Through ventilation must be provided

for all parts of the boat so that moisture can also be removed as water vapour

in the air stream

Care must be exercised in selection of the timber used to ensure the

rejection of material which has been infected in the standing tree. Premature

decay of oak due to prior infection with Polyporus aulphureus sometimes gives

trouble. Prior infection of Columbian pine is much more common and, as it is

-difficult to detect, this timber should be heat sterilised before use.

All wood for boat construction should be thoroughly seasoned and then

held under dry, well ventilated storage conditions until it is needed for use

Proper maintenance will do much to prevent development of decay. The

broad principles which must be followed are to prevent entry of water into the

boat by every possible means and to maintain effective ventilation to remove

moisture which does penetrate _{Some of the problems which arise are discussed}

in a general way in the FPR Bulletin No.31 and in greater detail in Duff's

papers and in Dawson's book

Remedial measures

When decay caused by Basidiomycetes does occur the only safe remedy is

usually to remove the decayed portions of the affected members together with

at least a few inches of adjoining, apparently sound wood- Defects permitting

entry of moisture should be located and remedied, To facilitate inspection

and subsequent drying paint should be removed around the area of the decay.

Ventilation should be improved, even if only a temporary basis, to promote

drying. For rapid, controlled drying, dehumidifiers using a refrigeration

system of air drying offer many advantages.

Full use should be made of wood preservatives when carrying out repairs.

In some cases it may be advantageous to obtain deeper penetration of

preservatives than can be achieved by surface treatment methods. Tools are

available for treatment of bolt holes under pressure or for the injection of

a paste of preserving chemicals below the wood surface. With very wet

softwood if drying can be postponed for a month or two after treatment, copious

application of strong (25-30 per cent) solutions of soluble borates ("Polybor")

-in hot water may achieve penetration by diffusion. For dryer timbers, the

proprietary preparation "Woodtreat" offers an easier method of achieving

penetration. A few weeks must be allowed for diffusion of the chemical, The

treated wood should then be cleaned down and primed with a leaf-forming

aluminium primer to minimise risks of possible discoloration of the new paint.

Superficial softening of the outer face of hull bottom planking is

likely to be due to decay of the soft rot type. If such decay has not

penetrated sufficiently to cause marked weakening it should only be necessary

to dry off the wood surface and then apply wood preservatives containing

copper salts.

Literature

Timber properties

FOREST PRODUCTS RESEARCH LABORATORY. A Handbook of Hardwoods, HMSO, 1956,

A Handbook of Softwoods. HMSO, 1957,

AV. THOMAS- Timbers used in the Boat Building Industry, FPRL 1964.

Agencies of deterioration

W.P,K. FINDLAY. Timber Pests and Diseases, Pelgamon Press 1967.

TIMBER RESEARCH AND DEVELOPMENT ASSOCIATION. Timber Pests and their Control,

1964-Preservatives

BRITISH WOOD PRESERVING ASSOCIATION and, .TIMBER RESEARCH -AND DEVELOPMENT

ASSOCIATION Timber. Preservation,

TINDLAY. The preservation of Timber,. Adam VCIaarles Black 1962

(out of print).

-Plywood and gluLam construction

FOREST PRODUCTS RESEARCH LABORATORY. Gluing Preservative Treated Wood,

Technical Note No.31, General

C. DAWSON. _{The Hull (Practical Boat Owner Maintenance Series), George Newnes,}

1967.

M.G. DUFF. _{Decay in wood-built boats, Ship and Boat Builder, 1951-2,}

5, 173-7; 246-51; 427-43.

J.G. SAVORY and D.F. PACKMAN, _{Prevention of Decay in Wood in Boats,}

Forest Products Research Bulletin No.31, HMSO 1954.

TIMBER RESEARCH AND DEVELOPMENT ASSOCIATION. _{Timber & Plywood in Boatbuilding}

(a series of leaflets on specific topics).

J.G. SAVORY and A. EAVES. _{Decay in Scottish Fishing Boats, FPRL 1965.}

Acknowledgment

This paper is published by permission of the Ministry of Technology.

-CONTRIBUTION BY P.E. WHITE

TYPES OF STAINLESS STEEL

"Stainless" or more appropriately "corrosion resisting" steels contain

12% or more chromium as the most important alloy addition. The most corrosion

resistant compositions are _known as austenitic steels and contain at least

la% Cr, and 8% Ni (popularly referred to as "18-8"type) and for the extra

performance demanded in marine applications austenitic steel containing 2 - 3%

molybdenum in addition to Cr. and Ni. is considered essential, This grade

is specified as Type 316 (previously En,58.J) in B.S. Specifications and

popularly known as "18-10-3" type,

Stainless steels without nickel and molybdenum are seldom appropriate

to marine application and are referred to as martensitic and ferritic grades.

Active and Passive Behaviour

In the paper by 11±, Ball the galvanic series of metals was shown but it

did not show that stainless steels are not always in the more noble (passive)

state. Under some conditions which may arise in sea water imwersion situations

less noble (active) behaviour may occur. This fact is illustrated by the

severe corrosion of stainless steel woodscrews that can occur when driven into

salt water saturated timbers of a hull. The action of the chlorides in the

sea water and the absence of oxygen at the metal-wood interface causes

penetration of the normally passive oxide film on the steel and as it cannot

reform spontaneously as it would in air, active (corroding) behaviour occurs.

Screw heads may remain unattacked but the corrosion where unseen can reduce

the holding power of the fastener.

-Bronze and nickel-alloy (Monel) woodscrews never give rise to this

particular problem. Consequently they are recommended for use below the water

line, Stainless steel fasteners however give excellent service _{on applications}

on deck and all top side fittings where strength, durability and good _appearance

are an essential combination,

Corrosion Resistant Copper Alloys

In discussing bronzes for marine use it is important that no-one should

regard "manganese bronze" mentioned by previous speakers, _{as anything but}

a brass with high strength and subject to dezincification in sea-water like

ordinary brass.

For screws, bolts and nuts, silicon bronze, phosphor bronze and aluminium

bronze have proved to be sea worthy alloys for applications above and below the

water line.

To improve the surface appearance of bronze fasteners, nickel chromium

plating is a suitable durable finish and matches similarly plated gunmetal

components.

Functional and Maintenance Values

Corrosion resistant fasteners provide more than sound load bearing joints

for boat structures, _{In applications where easy release and thread rotation is}

essential for access, adjustment and maintenance, time and money are saved by

correct selection and application,

The accompanying illustrations show the condition of Butterworth tank

cover stud bolts on the deck of "Esso Oxford" after two years voyaging. The

contrast between the condition of carbon steel studs and 1811013 stainless

steel is very obvious and very significant to the crew.

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