I’m thinking of importing some RV parts (control surfaces) which have been built in America without internal corrosion proofing. However corrosion-proofing is generally recommended in the British climate.
One option might be to just use ACF-50 regularly, but are there any methods of corrosion-proofing after the event. e.g. might it be possible to dip the parts in Chromate commercially, or some other similar option?
You are on an uphill struggle to corrosion proof untreated parts without opening them up , applying the full surface treatment ( allodine +etch primer ). This would have structural implications as removing rivets and replacing them is not always easy.
As you say just spraying the parts with a corrosion inhibited is probably the best option, ACF-50 seems to be one option, I have had a lot of success with CorrosionX ( as have the US Army who published a glowing report about their cost savings with the product ).
There are a lot of products on the market and while being a CorrosionX fan I would think that any of the products listed for use in Cessna’s SIDS document are likely to provide a good level of corrosion protection.
The last thing that needs saying is don’t over apply these products as they are capable of finding even the smallest gap in rivets and lubricating them untill they work loose.
Thanks for your advice. I’ve contacted the LAA regarding the parts that I am wanting to import and they have confirmed that they will accept them (if otherwise well built).
I had a reason to look into primers a few years ago and was told by a paint specialist that the zinc chromate stuff is only used by people wanting to maintain the authentic WW2 look, and that epoxy outperforms it in every department, including toxicity for which zinc chromate is famous.
I’ve been using ACF50 for about 15 years and it really is amazing stuff, but as A&C says it will find its way out past any rivets which have been done less than 100%. I am not sure that it actually loosens them though. It’s an interesting topic; if I was doing airframe assembly I would wet-rivet it with PRC sealant, so there is no gap at all where water could seep in. Messy but extremely effective. But in this case, with pre-riveted parts, it is too late for that.
Peter wrote:
had a reason to look into primers a few years ago and was told by a paint specialist that the zinc chromate stuff is only used by people wanting to maintain the authentic WW2 look, and that epoxy outperforms it in every department, including toxicity for which zinc chromate is famous.
Epoxy is definitely better (especially if weight is not a major issue) but zinc chromate is amazingly good. Relevant to this thread is that with zinc chromate you don’t have to cover every surface to provide protection. A dusting of the stuff has protected a pair of now uncovered wings hanging on my wall since 1946, and they still look pretty good despite having been exposed to the elements and God knows what else for decades. The paint is so thin that you can read the Alcoa 24ST markings through the paint, but that’s enough. They’re still airworthy and will be built up into a ‘new’ set of wings eventually.
The dust produced by sanding zinc chromate is toxic. It’s not otherwise, so there’s no major issue unless you later decide to sand & refinish the parts.
kwlf wrote:
I’m thinking of importing some RV parts (control surfaces) which have been built in America without internal corrosion proofing.
Control surfaces on the RVs are rather quick to built (considering it’s not a 3, 4 or 6, they taking considerably longer). When they corrode, you just build new ones. Will they? probably, but it depends on too many factors to say how fast, could take months, could take decades.
I don’t think the main problem is the aluminium itself. It’s all alclad (the outer layer of pure aluminium could corrode, but not the core high alloy material). But there are steel parts riveted onto the aluminium. They are probably powder coated, but not necessarily, and bare black steel against bare aluminium is never a good idea. The steel will rust between the mating surfaces, and by doing so it creates a the perfect environment for corrosion of the aluminium also. If they are powder coated, it’s better, but basic powder coating is only for looks. Even if it’s powder coated, the aluminium isn’t, and the mating surface is still almost perfect for corrosion. Steel parts should be primed with a zinc rich primer, preferably epoxy, but there are other stuff that will do the job just as well. There are special compounds you can use also (if you have bare metal against metal). The best known is Duralac. It’s barium chromate stuff that I use on all these things, but RTV will also do.
Corrosion needs 3 things; something to corrode (metal), oxygen (air) and electrolyte (water). Removing one, and it wont corrode. It’s therefore also important to prevent water from accumulating by making drains and so on, particularly if you have bare metal parts.
Silvaire wrote:
wings hanging on my wall since 1946
Exactly how old are you? 
I’m not 72, neither is my hangar, but the wings and paint are I bought the spare wings from a guy in Arizona who had about 10 of them with various states of minor damage. These two came from different planes, as did the rest he had. In the old days people used to buy new individual wings instead of repairing the old ones, and immediately postwar there were a LOT of training hours flown in the US. Lots of opportunity to ground loop a tailwheel airplane with a GI Bill student! As an example my plane #1 (also made that year) was flown 500 hrs between March 1946 and the end of summer, often 5 or more hours per day. The wings currently mounted on my plane aren’t original either…
Happily all that history and 6000 planes built leaves a lot of spare parts floating around, many of which have been protected by zinc chromate for 70 years or so. They were sprayed after assembly, I’ve seen old film of them doing it, but there aren’t a great many closed sections in a fabric covered wing – just the leading edge which wasn’t internally primed. Unfortunately this is also a perfect location for a bird’s nest, so replacing the leading edges is par for the course when building up a set of ‘new’ wings.
The fuselage on the same plane was completely rebuilt 20 years ago and was epoxy primed internally at that time. I think a trained midget was employed… that plus one uninstalled skin to allow internal access. It will last for a long, long time.
Like Silvaire I have to deal with some old aircraft and an currently overseeing the re-build of a Korean era warbird, it has the old green zink chromate primer protection in the inside and the lack of corrosion in the airframe is remarkable with only the dissimilar metal joints showing any cause for concern.
Moving on, I don’t think since I got into this business in 1976 I have seen any of the old zinc chromate primer being used it being superseded by 2 pack epoxys of one flavour or another.
One thing that seems to have gone out of fashion is wet builds, when I started the UK industry was obsessed with putting skins together with chromate jointing compounds ( JC4 if my memory is correct ) that did not set. I am told that this practice has been abandoned because it was found that the jointing compound was reducing the swelling of the rivet when driven and providing a libricant for the rivet to work loose. This must be a long term problem as I was around during the final days of the VC10 and there was no evidence of rivets coming loose despite the gallons of jointing compound used by Vickers during construction , perhaps the VC10 airframes that did another twenty years with the RAF would tell another story.
It would seem that modern construction just requires a surface preparation with alodine and then a light coat of epoxy etch primer before the fasteners are driven in, the in service protection being fluids such as ACF50 and CorrosionX applied during heavy maintenance
