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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 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 |
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 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. |
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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 CONVERSIONWith 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 |
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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. |
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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 |
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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 The Autocolour primer is 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/ And the MSDS
http://www.hex.co.uk/MSDS/ppg/ENG/F397.pdf |
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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 |
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. |
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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. |
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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. |
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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 |
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