Surface rust is a term used by motor and caravan dealers to attempt to minimalise the evident corrosion in chassis and other metal componentry, to avoid warranty. All corrosion inherently weakens metal structures, it can be prevented with Hot Dip Galvanising and sacrificial anodes, coating in any substance that prevents surface exposure to air (fish oil). Cold applied zinc paints and coatings such as Gal Met are rarely effective in preventing corrosion as it is usually applied without correct preparatory actions such as surface cleaning, priming, etc.

It is still on the surface is it not. I put it to you that it is a term invented by sales and suppliers, to be able to dismiss what is just plain rust and disguise it with the word surface rust.

And the point of this thread is ,.. ?

Still bored Iana?

uBeen fairly busy over the last two weeks, so no. Back to the subject. It was "The Belmont bears" experience in "General" that mentioned surface rust again, and that is what promoted the my thoughts & question.

Going back to the late sixties, I was employed to do sandblasting and crack checking as part of my apprenticeship training. Metal items were sandblasted to remove corrosion (rust) then to be inspected, plated or painted and put back in service.

Those steel parts that were sandblasted, which removed the corrosion, but also removed the oxidized coating (natural corrosion protection) would, when left over night develop a golden sheen, this was called surface rust (corrosion) and this is what I think is the correct meaning. Parts were sent asp off for cadmium plating.

While you can have light corrosion, or heavy corrosion, its still starting at the surface. And the only way to see how deep it goes is by sandblasting or a similar method that gets all of the oxidization out. Believe me I did a lot of corrosion removal during those years. The inspectors would insist on 100% removal.

So I think the borrowed phrase "Surface corrosion", is being used as a means, where we if we take our van for example, back for warranty etc. because the galvanizing has failed and corrosion has set in, as a means of dismissing our claim.

-- Edited by iana on Tuesday 3rd of December 2019 09:57:06 AM

iana wrote:

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uBeen fairly busy over the last two weeks, so no. Back to the subject. It was "The Belmont bears" experience in "General" that mentioned surface rust again, and that is what promoted the my thoughts & question.

Going back to the late sixties, I was employed to do sandblasting and crack checking as part of my apprenticeship training. Metal items were sandblasted to remove corrosion (rust) then to be inspected, plated or painted and put back in service.

Those steel parts that were sandblasted, which removed the corrosion, but also removed the oxidized coating (natural corrosion protection) would, when left over night develop a golden sheen, this was called surface rust (corrosion) and this is what I think is the correct meaning. Parts were sent asp off for cadmium plating.

While you can have light corrosion, or heavy corrosion, its still starting at the surface. And the only way to see how deep it goes is by sandblasting or a similar method that gets all of the oxidization out. Believe me I did a lot of corrosion removal during those years. The inspectors would insist on 100% removal.

So I think the borrowed phrase "Surface corrosion", is being used as a means, where we if we take our van for example, back for warranty etc. because the galvanizing has failed and corrosion has set in, as a means of dismissing our claim.

-- Edited by iana on Tuesday 3rd of December 2019 09:57:06 AM

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 IF the rust has penetrated the galvanising it is DEFINITELY NOT SURFACE RUST, it is corrosion.  Could be caused by lots of things,,, poor preparation, poor galv quality, not rolled 360 etc to get gal everywhere, scratched or stone damaged surface in use, or too thin a galv coat worn down by dust.

One of my boat trailers in Darwin was severely sandblasted from dirt road dust (not stones), but I paid full tote odds for a good quality trailer so gal lasted OK. Other well known brands would have failed for sure.

-- Edited by Baz421 on Tuesday 3rd of December 2019 05:24:56 PM

Some slightly dodgy Googled definitions might be useful at this point -

'Corrosion is the chemical or electrochemical reaction that causes an engineered material to disintegrate as a reaction to its surroundings. Metals are the usual materials that undergo corrosion. It is a gradual process with the elements eating away at the materials making it deteriorate and break up because of the oxidation of the metals as a chemical reaction to an oxidant, usually oxygen.

While it is common in metals, it can also happen to other materials such as ceramics and polymers, but it is called by another name which is degradation. Exposed materials are prone to corrosion, and they can be just a crack in a small area, or they can be concentrated in a larger part. The discoloration of painted materials, when paints fade due to neglect, is an example of corrosion. Even without a visible oxidant, materials can still corrode when left in the open air.

Rust, on the other hand, is a type of corrosion which happens to iron and its alloys. When iron reacts to water or the moist air, iron oxides are formed and cause the material to corrode and rust. It is caused by oxidation and moisture and not by chemicals. This happens when impure iron comes in contact with water or moist air and oxygen or other oxidants, such as acids, and forms rust.

Other factors or oxidants that can cause rust are salt, sulfur oxide, and carbon dioxide. Rust comes in different forms, the most common of which is the red rust which is formed by red oxides. Chlorine in water causes the formation of green rust. Like most corrosive processes, rusting is a gradual process. After some time, if the material is not treated, it will disintegrate, and everything will turn to rust making it unusable.'

So surface rust is the cosmetic (non-structural) corrosion of iron (or usually steel in our case) caused by exposure to moisture and oxygen, or rarely another oxidant. It is easily treated by removal of the rust and any source of chemical oxidant and/or applying a protective coating.

I agree with the metalurgist. The term is an indicator of the level of work needed. It is used all the time, in shipbuilding for example, to indicate the level of rectification work required by a dockyard - routine preventative maintenance in this case. Or in the case of certain ex-American rust buckets held together only by many layers of paint - almost a total freakin re-build.

As far as automotive panel work goes, surface rust starts from the outside and eats its way in. Those small bubbles you sometimes see is rust that has eaten its way from the inside surface right through to the outside surface and pushes away the paint as the oxidised material takes up a bigger area than the non oxidised material. It has to go somewhere and the layers formed from the penetrating side are stronger than the paint layer on the outer surface, so it pushes it out in a bubble before it finally breaks through the paint to expose a visible rust patch. Dig at the rust patch and you will soon see just how large the affected area is on the other side of the panel. What looked like a tiny hole is in the middle of a large area of very thin material.

Surface rust wipes off with a rag and a weak solution of phosphoric acid, serious corrosion will not wipe off so it is much deeper than surface rust.

T1 Terry

T1 Terry wrote:

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As far as automotive panel work goes, surface rust starts from the outside and eats its way in. Those small bubbles you sometimes see is rust that has eaten its way from the inside surface right through to the outside surface and pushes away the paint as the oxidised material takes up a bigger area than the non oxidised material. It has to go somewhere and the layers formed from the penetrating side are stronger than the paint layer on the outer surface, so it pushes it out in a bubble before it finally breaks through the paint to expose a visible rust patch. Dig at the rust patch and you will soon see just how large the affected area is on the other side of the panel. What looked like a tiny hole is in the middle of a large area of very thin material.

Surface rust wipes off with a rag and a weak solution of phosphoric acid, serious corrosion will not wipe off so it is much deeper than surface rust.

T1 Terry

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Yep this is inevitably what happens when you just seal (paint or otherwise) over rust that has not been properly treated.  One benchmark for a quality chassis builder is that they will have a thorough and effective process in place for treating then immediately coating the built chassis.  The van builder should have a process in place to effectively treat and protect any damage they do to the chassis coating.

I plan on doing the factory tour offered by many of the 'quality' builders to check this, as well as the quality of the hidden plumbing/electrical work, before I drop my hard earned on one of their obscenely priced products.

Sal and John wrote:

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Not a metallurgist but an experienced design engineer.    It's all corrosion.   At the stage of that light stuff, we are looking at microscopic pits which develop miniscule chemistry as one sees in a battery and the normal ion transfer starts off.   Untreated it just gets worse and worse.

One of the main concerns (with structurally loaded components) is the likelihood of fatigue cracking initiating in association with the corrosion pitting.

Plenty of info in the textbooks and on the net.   For instance,

https://www.nature.com/articles/s41598-018-26340-5

If that link gets a bit too boring, just think micro sized batteries at work damaging the surface structure.

 

-- Edited by Sal and John on Sunday 8th of December 2019 09:34:44 PM

-- Edited by Sal and John on Sunday 8th of December 2019 09:36:50 PM

-- Edited by Sal and John on Sunday 8th of December 2019 09:37:11 PM

-- Edited by Sal and John on Sunday 8th of December 2019 09:39:06 PM

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One of the more serious side effects of using the vehicle body as the negative return path to the battery (well, it's actually the other way around but that will just cause a migraine trying to explain why) There are always stray voltage potential differences and using the body as the negative circuit just adds to the problem.

No idea if it's still around, but there was a rust reducing kit for cars on the market back in the '80s and 90s that linked all the steel panels together using earthing straps, then there was some sort of sacrificial anode idea and frequency pulsing ....... sounded like snake oil to me at the time but the electrically bonding all the body parts to each other did make sense.

 

T1 Terry