CORROSION PREVENTION, PRIMERS AND WET CONSTRUCTION

 

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This article was first written in the mid 1990s. It is not intended to be the start of a discussion, or an instalment of Primer Wars! It is really some back ground information to help British & European builders figure out what makes sense, and to see through all the confusing information that sometimes emanates from across the Atlantic.

 

Please take what is written here in the spirit it is intended – as information to help you determine the best way ahead in your own particular circumstance. What has worked for me may not be suitable for anyone else!

 

Corrosion prevention in aircraft structures is a topic that demands much more attention in the UK than in the USA.  Our damp climate can be fatal to metal as well as wooden aeroplanes.  One of the best ways to prevent corrosion is to adequately protect the structure when it is built.  There are several methods of achieving good protection, none are cheap, but time, effort and money spent while building will pay dividends in years to come.  The main processes involved are plating, surface conversion (including anodising), the application of a primer and/or paint scheme, and the use of wet construction techniques.  Let us first consider the base metals we are interested in, Aluminium and Steel. Later sections will look the methods for providing corrosion resistance and what can be done practically done by the average homebuilder.

 

Aluminium  Pure aluminium is reasonably corrosion resistant.  A very thin surface layer oxidises and prevents the underlying metal from corroding any further.  Unfortunately pure aluminium is quite soft so is alloyed with copper and other elements to greatly improve its strength and stiffness.  The alloy then becomes corrodible (just our luck!).  A thin skin of pure aluminium (about 0.003" thick) can be rolled on to alloy sheet and plate to make it more corrosion resistant.  This is known (in Britain especially) as Alclad and is the type of material that Van's use for all thin sheet.  When using Alclad, components have to be a gauge thicker than when using non clad sheet for the same strength.  Thus, components made from clad sheet are heavier.  If the parts are anodised the thinner gauge can be used, saving weight.  Bar and thicker plate are not normally available clad so another corrosion resisting process must be used.  In all cases a corrosion resisting primer is often used.

 

Steel  The propensity for steel to rust is well known.  There is no steel equivalent of alclad so corrosion protection must be provided by painting or plating.  Corrosion resistant (stainless) steels are available but are expensive and are usually only used when their special properties are really needed, ie firewalls.

 

 

Looking at the generally available corrosion prevention techniques we have plating, surface conversion, primers and wet construction techniques.

 

PLATING

 

Plating is an electrochemical process where a metallic coating is deposited on to the base material. The plating provides the corrosion resistance. For the plating to stick the base has to be very clean, thus, preparation costs are a significant proportion of the total.  The equipment required for the whole process is expensive and some fairly nasty chemicals are involved.

 

Two types of coatings are used, sacrificial coatings and barrier coatings.  In the first case the coating, such as cadmium or zinc on steel, protects the base metal by gal­vanic action (thus zinc plating is know as galvanising).  That is the cadmium or zinc will corrode before the steel. At any break in the coating the plating will suffer sacrificial corrosion thereby protecting the steel.  Eventually all the cadmium or zinc will be removed and the steel will start to corrode.  This usually takes many years.

 

When a barrier coating is used, such as chromium on steel, the coating acts purely as a barrier to prevent the base metal being attacked and relies on the integrity of the coating for complete protection.  Hard chromium is not always used for corrosion protection but some times for wear resis­tance, for example on undercarriage oleos.

 

 
SURFACE CONVERSION

 

With surface corrosion the properties of a thin layer of the base metal are changed to provide corrosion protection. Surface conversion coatings are formed by the action of a chemical to change the surface layer of the metal to a metallic oxide or salt, sometimes with the help of an electric current.  The resultant layer has superior corrosion resistance properties to the base metal and also pro­vides a very good key for subsequent paint finishes.  Two examples of this process used with aluminium alloys are anodising and alocroming. 

 

Anodising  When aluminium and its alloys are anodised a thin, hard, oxide film is formed on the surface.  A bath made from metal is used to allow the components to be suspended in a converting liquid and an electric current to be passed.  By adding dyes various colours can be produced.  Blue, red, gold and black are common, grey is the natural colour.

 

Alocroming  Alocrom is a trade name for a process in which an acidified chromate is brushed on to the aluminium to form a chromate salt, it is sometimes known as Alodining.  The preparation used was invented by ICI and is called Alocrom 1200.  There are now several similar products available, Alodine is a US product that does basically the same thing. A corrosion removing liquid called Deoxidine 624 is often used to prepare aluminium before alocrom is applied.  Both these products contain some fairly nasty chemicals and require the safety precautions laid down on their data sheets be closely followed.  In particular Alocrom must not be kept mixed (it is supplied as two separate liquids) for more than 24 hours otherwise toxic fumes are given off.

 

 

EMBRITTLEMENT

 

Anodising and plating can cause embrittlement of the base metal.  This can adversely affect the fatigue strength of a component.  A few years ago  there was a scare that spars made (and anodised) by Phlogiston were prone to fatigue cracking.  This has proved to be unfounded, as Van's noted in an issue of Rvator at the time.  A very slight reduction in fatigue strength (BAE Systems use a 5% reduction in their calculations) more than compensates for the reduction in strength due to corrosion during the life of your aeroplane if the anodising had not been there.  If the designer intends anodising to be used from the outset allowances can be made in the design.

 

USE OF ANODISING AND PLATING

 

Because these processes are complex and require expensive equipment, homebuilders must contract out to specialised firms.  There are several around the country who are CAA approved.  Most will consider small, one off, jobs.  A minimum charge is often made, perhaps £50 to £100.  To Cadmium plate the engine mount and roll over cage of an RV-4 will probably cost several hundred pounds.  The more items done together the cheaper it becomes.  Cost is generally proportional to surface area.  It is not a good idea to plate Spring Steel com­ponents, for example undercarriage legs, as the loss of fatigue strength due to embrittlement can become significant.  Some people say that welded components should not be Cadmium plated due to hydrogen embrittlement of the welds (the same may be true for welded components).  I have not been able to find any authoritative sources on the subject.

 

PRIMING AND PRIMERS

 

Van's insists that all parts made from 6000 series aluminium alloys are primed to ensure their longevity.  Primers come in several different types ranging from simple lacquers through two pack epoxy paints to acid etch primers.  Most are based on metal chromates, usually zinc, to provide the necessary level of corrosion protection.  If parts are made from clad sheet or have been plated, anodised or alocromed an epoxy or normal paint is probably sufficient.  Other components are likely to benefit from treatment with an etch primer.  Effectively using an etch primer does in one process what alocroming and epoxy priming will achieve in two.  If a non aerospace primer is used considerable savings can be made.  I am using an autocolour (previously ICI) primer intended for use by commercial truck body builders that was recommended to me by a friend who runs a coach building business.  I cannot vouch for its long term performance but it is fairly easy to work with and not nearly as toxic as Alocrom.  I have, however, used Alocrom for my RV-4 spars.  Etch primers can be used on steels and other metals as well as aluminium.

 

 

 

It is essential that all surfaces to be primed are as clean as possible to ensure good paint adhesion.  At the same time do not under estimate the part played by your finish coats of paint.  By all means use automotive products but be sure that they will be flexible enough to withstand the amount your airframe will bend.  I would suggest that it is a false economy to scrimp on primers as it could, potentially, be very costly in years to come if the wrong choice is made. 

 

I am priming all the internal structure of my RV
whether it is made of clad sheet or not.

 

 

WET CONSTRUCTION TECHNIQUES

 

Few people seem to be familiar with wet construction techniques or the part they play in keeping the dreaded metal eating worm out of your pride and joy.

 

The basic theory is that if you put a joint­ing compound between all the parts you rivet together then they cannot rub against each other nor will any moisture get into the gap between them.  It all sounds fairly simple.  In practice everything gets covered in yellow gunge which makes the whole job much more difficult.  However, most corrosion will start around rivet holes. The use of wet construction tech­niques will make it much harder for the corrosion to start.  Three types of jointing compound are available, Duralac, JC5A and Polycast, all are based on zinc chromate paste. 

 

 

 

 

 

 

Duralac is a thick yellow paint that dries in time.  JC5A is known as a pigmented varnish jointing compound.  It never really sets but will go tacky and less useable after about 8 hours.  Polycast is known as a chromated sealing and filleting compound.  It is a 2 part mix which must be used before it sets. Polycast is the best to use but is the most expens­ive.

 

Alocrom, primer and paint from Light Aerospares
(Address in Popular Flying/Light Aviation)

 

The Autocolour primer is
P565-625  Primecoat and
P275-124  Standard Activator

As the paints division of ICI has been taken over by PPG I don’t know if this is still available. The nearest I have been able to find on the PPG web site is PPG F397, and its associated activator F368. I have not used this product and do not know if it is suitable for our kind of application. Please read the data-sheet

 

PPG F397 https://buyat.ppg.com/Refinish_SSA/Common/
ShowPdf.aspx?AppId=2&BrandId=3&LanguageId=14&
IntDialCode=44&PdfId=8589&ProductCode=F397

 

And the MSDS http://www.hex.co.uk/MSDS/ppg/ENG/F397.pdf

 

 

PRACTICAL PRIMING & WET CONSTRUCTION

 

Data Sheets  Just about all corrosion inhibiting compounds are toxic to one degree or another.  Insist that your suppliers provide you with a data sheet on the product you are purchasing.  Follow the safety precautions outlined - if a face mask is called for use one.  Chromates have been found to be carcinogenic in certain circumstances so take care.  Some of the products will decompose to highly dangerous substances if kept mixed too long - so don't exceed the recommended storage periods.  Be careful how you dispose of the waste.  As well as safety precautions the data sheets should also give advice on application and surface preparation.

 

I have found the Hobbyair products to be very useful and use it when ever I am painting.

http://www.refinishingonline.com/hobbyair.htm

They do a 240v version and also provide a full face mask. If there is any demand I will see if I can add the Axis line of products to Gloster Air Parts range, please call or email if you are interested.

 

Before I move on to “full up” Alochroming I have recently been using “Touch’n’Prep” pens. They are like large felt tip pens and are loaded with Alodine, except there is no need to wash it off once done. They are approved by the US Air Force (technique sheet on-line somewhere). I have not yet found an outlet in the UK, and they are likely to be in the £40 range, but are really useful for giving something a quick coat of corrosion protection.

 

 

PRIMING - Alocrom

 

As you might expect, as Alocrom is about the best corrosion inhibitor it is also the nastiest to work with.  It is supplied in two parts that have to be mixed in equal quantities before use.  If the mix is kept for more than 24 hours it will start to give off some highly toxic gases so throw away what you do not use. Please don’t throw it down the drain, neat.

 

Preparation - The cleaner the metal is before the Alocrom is applied the better the result will be.  All grease and oil must be removed along with any corrosion that may have already started.  Rubbing down with scotchbrite will remove any light corrosion and wiping with thinners or gun wash (cheap thinners) is one way of getting rid of any gunge.  There are cleaning compounds available, such as Deoxidine 624 - see below, which are designed to prepare metal surfaces before Alocrom treatment.

 

Application - When Alocrom is applied properly the metal takes on a golden tinge.  It is usually applied with a brush or sponge, but I have known of people rubbing it in with scotchbrite.  Once applied it is left to act until the metal has a light golden colour when the part is thoroughly washed in water.  If deeper protection is required the Alocrom can be reapplied.

 

Precautions - Rubber gloves seem like a good idea.  Avoid skin contact, be careful of splashes, and try not to get the stuff on your clothes.  Is all the hassle worth it?  Probably yes for non clad parts, if it is done properly, Alocrom protection will last for years.  Alodine is the same stuff but under another name.

 

Further Coats - A primer, such as an epoxy 2 pack primer or an ordinary primer should be used before a top coat is applied.

 

Preparation – Deoxidine  Deoxidine 624 will clean aluminium of just about any contaminant, including corrosion, prior to application of Alocrom or another primer.  It is specifically designed to be used with Alocrom.  The active ingredients are rather viscous so be careful when using it.  It is probably only worth considering its use for non Alclad parts.

 

 

PRIMING - Etch Primer

 

Etch primers are not usually as nasty as Alocrom, probably because their use is more wide spread and the paint manufacturers have spent more time in making them slightly more user friendly.  They do the job of Alocrom and an ordinary primer in one application so are much less time consuming and probably save money.

 

Preparation - Wiping with gun wash after a light rub over with scotchbrite is probably sufficient.  Again everything should be dry and free from oil, grease and corrosion.

 

Application - Etch primers are usually sprayed but can sometimes be brushed.  A second coat can often be added a few minutes after the first (but watch the weight).  Some etch primers are hydroscopic (take in water) and so have to be coated with another primer within a sort time of application (Water in the primer will promote corrosion).  Check when you buy and check the datasheet.

 

 

Precautions  As an absolute minimum use a facemask when spraying and change the filter regularly, but I would use the Hobby Air system described above. You only get one chance with your health.

 

Make sure there is some ventilation in your work/spray shop.  Check the datasheet to find out if there is a minimum temperature to apply the primer.  If your spray shop needs heating remember to heat the parts to the same temperature as the paint, ie heat everything for an hour or so before starting to spray.

 

PRIMING - Ordinary Chromate and 2-pack Primers

 

2 pack epoxy primers have better corrosion prevention properties than straight single pack preparations, but are also more expensive.  These products can be used on their own, but an acid etch product (Alocrom or etch primer) will give better corrosion protection if used properly.  There are no specific points to bear in mind that have not already been mentioned above.  As always good adhesion to the base metal is the key to long term protection so don't skimp on the preparation.

 

 

WET CONSTRUCTION

 

As noted above, the idea behind wet construction techniques is to exclude any moisture from a joint by filling any gaps with a jointing compound.  (Don't confuse this with sealing fuel tanks.  The processes are similar but different compounds are used and the aim is different, although when sealing your tanks you are protecting the joints as well.)

 

Three types of jointing compound are commonly used, they are all yellow, because of the chromate, and gungy.  In ascending order of protecting ability (ie the last is best), and also ascending cost, they are Duralac, JC5A and Polycast.  A brief description of each was given above.  By reading this far you have already accepted that using a jointing compound is a good idea.  In our type of aircraft Polycast is probably only worth using in important structural members ie mainspar, main fuselage frames etc.  JC5A is probably acceptable in most places, although, if Duralac is the only stuff you can afford it is far better than nothing!  A note of caution - do not be tempted to use ordinary jointing compound for your fuel tanks - the fuel will adversely react with the yellow gunge and may end up dissolving it away.

 

 

 

 

The time to apply the stuff is just before a joint is rivetted together.  Ensure that both mating surfaces have been painted and apply the jointing compound to one side of the joint - usually the part being attached as this avoids using excessive amounts.  Bring the two pieces together, wiggle slightly to bed down and cleco together.  Yes, your clecos will get covered as well.  It may be difficult to locate the rivet holes through the jointing compound, a cocktail stick comes in useful here. Anything harder, such as a piece of locking wire may scratch the paint.  Dob some compound into the rivet holes, insert a rivet and drive it.  Make sure you wipe the compound off the rivet tail as this makes it slippery and easy for the reaction bar to slide off.  A little compound should ooze out of the side of the joint if you have used the correct amount.  Wipe off any excess.  If you have not used enough there is not much you can do except use more next time.  Ordinary thinners/gun wash cleans up JC5A and Duralac well.  I believe MEK is used on Polycast.  Jointing compound does tend to get everywhere, but it will be worth it in 10 years time.  Have fun getting it out of your clothes & fingernails.

 

 

So what have I done?

 

You will be right to think that full wet construction is a real pain and adds hours to the construction time. For military aircraft that are operated in all weathers it is a necessity if they are to last 30+ years, especially if they are helicopters that hover over the sea all day long.

 

In aircraft that are usually operated in good weather and are likely to be hangared for most of their lives is it really necessary? The answer is probably no, but you’ll never really know for 20 years!  There are also compounds such as ACF-50 and Corrosion-X that can be applied and keep the dreaded metal eating worm at bay (but paint your aeroplane before applying these compounds!!!)

 

 

 

I have primed everything before riveting together. The easiest time to prime is at the beginning, the most effective is just before everything is riveted together – your choice. I have not used any jointing compound between skins & ribs or frames. But I did put a blob of primer in each rivet hole, I know it’s a lot of work, but corrosion often starts at rivet holes so I think this is worthwhile.

 

What about quick-builds? Van’s says they are primed with a wash primer – I think that is almost a waste of time. There are several UK quickbuilds that have suffered from corrosion. It is relatively easy to prime the inside of the fuselage, and you will have primed the back end as it was built, so that leaves the wings. Its almost impossible to get primer into the wing bays that have already been closed up, so I would suggest ACF-50 or similar once the aeroplane has been painted.