Re: Rust....

Don Anderson

During part of my career in the oil industry, I spent time as a corrosion specialist for downhole and surface oilfield facilities. I will attempt to explain how metal corrosion works especially as it applies to stainless steels and aluminum.
Corrosion of regular iron and steel alloys produces a corrosion product called iron oxide. Iron Oxide forms a very soft, porous and powdery coating that flakes off easily from the metal substrate. This constant flaking exposes more raw metal to oxygen which results in further corrosion. The process continues till all the iron is converted to iron oxide. However metals like stainless steel and aluminum resist corrosion by a proceed known as "passivation". Passivation works because as the metal corrodes (reacts with oxygen) at its surface, the resulting corrosion product forms a very molecularly tight, hard clear protective coating on the surface of the metal which excludes oxygen which then prevents further corrosion. In the case of Aluminum, the corrosion product is know as aluminum oxide or Alundum. Alundum is extremely hard which is why it is used in grinding wheels and sandpaper. The passivation process only works if there is a continuous supply of oxygen to replenish the protective oxide layer as it gets damaged or is worn off. Stainless steels especially the lower grades will corrode if one installs aluminum or SS in an oxygen poor environment.  Ask any boat owner especially if the boat is kept on saltwater, who has ever tried to remove a ss bolt from an aluminum fixture after years of use. 
As you know, stainless steel comes in many grades depending on their alloy composition and Chromium content. The lower grades like 304 have a low Chrome content and do exhibit a tendency to show some surface rust. It doesn't affect their serviceability and is cosmetic only. The higher grades like 316 or 18-8 do not exhibit this trait. I suspect 304 is the grade that AP uses for the counterweights. If you clean the surface rust off with a cleaner, you will remove the protective oxide layer and the metal surface will oxidize again until the protective layer reforms. Unfortunately this re-oxidation process will result in the reappearance of the rust look. Just the nature of the beast! If one is looking for a totally non corroding finish, one would need to look at the higher stainless grades or if cost was no object, Monel. It really all comes down to cost and what is the intended use of the finished product.

I hope this has been of some help.

Don Anderson


On Wed, 2/22/17, 'Joseph Zeglinski' J.Zeglinski@... [ap-gto] <ap-gto@...> wrote:

Subject: Re: [ap-gto] Rust....
To: ap-gto@...
Received: Wednesday, February 22, 2017, 3:00 PM


    Why are
we talking about CHROME? The rust problem has to, do with

Actually, I would be interested in someone at AP explaining
just WHY ...  SS rusts at all? I thought it was an alloy
impervious to rust. Can’t be just the iron content since
it is  tightly “alloyed” into the mix. Maybe the alloy
chosen for the cwts, isn’t a terribly good one to begin
with – perhaps better for its weight to size (density),
rather than rust susceptibility.


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