Corrosion...Rust...Cancer

[stllookn]

Are there any corrosion specialists from GM, Toyota, Volvo, the marine industry...Navy...whereever that frequent this board? It is time to weigh in on a discussion about the best way to protect body steel for the long term. By long term, I mean a 20+ year finish. I am not totally convinced that rust conversion products like osphos work in the long term, nor am I convinced that a wire brush and Hammerite will stop rust without getting down to "white" metal first. Any thoughts? I know this might be controversial for some of us...just looking for answers here.

[Southpa]

I work for a marine engineering firm during winter months. Its basically contract work and involves non-destructive thickness measuring of deck plates etc. on our local ferry fleet when in drydock for refits. A lot of these boats were built in the early 60's and have spent their life in a saltwater environment.

I guess the basic rule of thumb is to make sure your metal base is extremely clean, almost to the point of being sterile. Also, paint applied is by electrostatic means where it actually bonds with the metal and is not just a "veneer" covering where air pockets can reside. But I've still seen lots of corrosion in my travels. Usually where the marine enamel has been damaged and it spreads from there.

So in order to maintain your metal you should be constantly on the lookout for damaged paint and make touchups/repairs to prevent corrosion from spreading.

Another area where rust can take hold is a result bad welding. A good weld has no air pockets or impurities (slag) inside and no undercut at fillet edges. Those are great places for water to collect and for oxidation to begin.

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Southpa
1970 GMC 1500 Custom
Victoria, BC, Canada



[This message has been edited by Southpa (edited April 09, 2002).]

[stllookn]

Southpaw...so what was used as an undercoat in marine applications? My experience was with lead based or copper based primers to either prevent water intrusion or was it to prevent galvanic corrosion or both? What is the principal reason for undercoat on automotive finishes besides color for uniform reflection? If primers are not impervious to water migration then they must only be for filling and color enhancement.

[Southpa]

Normal steel primers are applied to most surfaces, followed by epoxy toughcoat on hulls, bulkheads etc. of large ships. The epoxy is applied at around 10-20 mil. (thousandes of an inch) thick, has some spring and is very durable when hitting floating debris. They also used to use something called "magnacoat" which has a greasy feel and repels water like crazy. But that material is no longer in use. The primary function of your lead and copper-based hull coating is actually to prevent sea critters like barnacles, mussels etc. from growing. Its used mostly on wooden boats and is slightly toxic, so barnacle larvae and shipworms tend to avoid it. ALL boats have to have zinc plates or chunks of zinc attached to any hull steel (cooling plates, rudders, props etc.) to prevent corrosion of the steel in seawater. Electronegativity of zinc is higher (more attractive to electrolysis) than that of iron.

In order to prevent paint damage sharp corners are always avoided and fast drainage on decks is necessary. That is why you will ALWAYS see "mouseholes" cut into the corners of hull stiffeners etc. ANY rough surfaces where you end up with a non-uniform coat of paint are invitations to corrosion. So I guess primers help in filling as well as providing a good bonding surface for your topcoat.

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Southpa
1970 GMC 1500 Custom
Victoria, BC, Canada

[stllookn]

Southpa, thanks for the refresher...forgot about the critters and the cathodic protection. So, I think I will surf and try to find some really in-depth analysis of automotive finishes and how best to protect the sheet metal. It bothers me that there is the potential to wire brush the heck out of something and still have it rust under the paint. I don't like wire wheeling since it only seems to polish the rust (still see discoloration) which means it is not all gone. I am not convinced that osphos or rustmort or some of those products work since they don't provide the chemical composition and how they work or exactly what they do to the ferric oxide to render it inert. I guess I had better head to the old corrosion library at the U to figure this out.

[Southpa]

Hey, if you find a way to turn that oxide into gold let me know (our secret, lol!).

Rust neutralizing DOES work tho. It doesn't remove rust but converts it an inert coating that seals the metal against further rusting (ie. iron oxide to iron phosphate). Lots of different brand names like Rustmort, Zintex, POR-15 etc. very likely all from the same vat.

The only other option is eating away the rust with a mild solution of phosphoric acid.
That can be messy and dangerous tho and is mostly recommended for dipping and not painting on. The acid will also eat away good metal too so you have to be careful about that. You could end up with pitting etc.

On a side note, I don't think you can get rid of ALL the rust anyway. Unrusted iron does not exist in our atmosphere. I used to do high pressure water blasting (up to 11,000 psi.) for cleaning components at pulp mills, refineries, etc. When I cleaned the crap off a heat exchanger or some other component, and I'm talking about right down to the base metal, you can actually watch the surface rust form like magic! Now if you can find an atmosphere that has 0% humidity you might have a chance at applying primer before the iron oxidizes, lol.

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Southpa
1970 GMC 1500 Custom
Victoria, BC, Canada



[This message has been edited by Southpa (edited April 10, 2002).]

[StingRay]

Over the years from having worked in a few different industries and occupations here is my slant on the issue. Clean clean metal is a must. Blast or abrade to white or near white, use a phosphoric acid based metal prep to remove any residual corrosion and to etch the surface, use a phosphoric acid with zinc product afterwards to leave behind a zinc phospate coating which will give you a little galvanic protection under the primer, epoxy prime then do your filling and sandable prime on top of that. For any surfaces that will not be visable I use Zinga or Devcon Z cold galvanizing compound. It can be left uncoated and is very tough. If you get any damage to the coating it will still protect. These manufacturers claim that you can top coat with epoxies and I have. I have also just undercoated over the stuff and both have been succesful in day to day use with what amounts to 4 year survivabilty at this point. There are very real concerns about long term adhesion of top coats to galvanic type coatings or materials so I would never use such a coating on cosmetic parts of the vehicle as I would be concerned about adhesion of top coat issues up the road. It's easy to throw a little undercoating or black epoxy on the underside if it does let go but it would be a disaster on a cosmetic finish. On my truck I will be using these very processes and will be backside and underside coating all non cosmetic surfaces with zinc based materials. This will include the insides of rockers, cab corners, floor supports, door pillars, fleetsides and the cab floor inside and out. Oh yes NONE of this is cheap. Expect to PAY PAY PAY. Coatings $100-$200 per gallon (Canadian dollars) and $100-$200 per 5 gallon pail on phosphoric acid based products.

[This message has been edited by StingRay (edited April 10, 2002).]

[This message has been edited by StingRay (edited April 11, 2002).]

[stllookn]

Southpa and Stingray...that is what I was looking for. That gives me good base from which to work. Thanks guys. BTW...Southpa I used to design 60,000 psi pumps for cutting rock and other tough materials. I never had any success getting to "white" metal until we started using 35,000+ psi. I know what you mean...you can watch it start to corrode immediately! We did some tests on aircraft carrier decks stripping the anti-skid material. It worked fast but corroded quickly. The officers and maintenance guys used to get very excited about the speed and did not seem to care much about the corrosion. I guess they had another way to deal with that like phosphoric acid etching.