Carbon Contaimination of Stainless Steel

[UKCats]

During the fabrication of stainless steel pressure vessels, it is very customary to specify not to use grinding wheels or wire brushes that have been around carbon steel material. I have seen first hand the minute corrosion that can occur when this is not adhered to.

Question.... Can the carbon in carbon tipped tooling cause the same problem of flange faces??? Do stainless steel flange fabricators use a different cutting tool (i.e. alloy based) to smooth out the flange face???

 

[mcguire]

No, the contamination in carbon steels is from the iron, not the carbon. The rust which forms when the iron corrodes prevents the stainless from maintaining a passive film...excess of iron ions. If carbon were introduced into the steel at a high temerature, i.e. carburized it would be detrimental. This cannot happen at room temperature.

 

[UKCats]

So does carbide tipped tooling have no excess Fe that imparts on the stainless steel surface??? Is this due to the hardness of the tool???

 

[TVP]

Carbide tooling is composed of tungsten carbide (~ 80%) in a matrix of cobalt (~ 20%). The only iron present would be debris remaining from machining an iron-containing alloy.

 

[mcguire]

I agree with TVP ( again ). Tooling in general usually is too hard to impart iron to stainless, especially cutting tools.

 

[111121]

In a fabrication process contamination can occur from a variety of sources. eg: When machining, recirculated cutting/soluble oils/emulsions are often used and these are rarely filtered.
The workpiece holding clamps, jaws and fixtures/tables are made from steel or cast iron and can contaminate the stainless surface. Dust or sparks from an adjacent fabrication process can also contaminate a surface and initiate corrosion.
I was once told "if it is worth building it out of stainless, it is worth passivating when you are finished". This involves the informed application of a dilute nitric acid based fluid or paste to a finished surface to ensure any film is (as McGuire Dec12 stated),passive.
Passivating or "pickling" proceedures are listed in ASTM A 380, with immersion in a 10% HNO3 + 2%HF bath at 50deg C being a popular proceedure for small fabricated austenitic stainless steel components.
For smaller jobs, your nearest Stainless steel supplier should be able to provide you with a paste and directions for use.

TJA

 

[kenvlach]

To 111121,
Re: "Passivating or "pickling" proceedures are listed in ASTM A 380, with immersion in a 10% HNO3 + 2%HF bath at 50deg C being a popular proceedure for small fabricated austenitic stainless steel components."

I'm curious. Is there any chance that these fabricators have fully annealed the SS before pickling? (Or is it all 321?)
Otherwise, this procedure is counter to ASTM A380-96.
See Paragraph 5.2.1 and Table A2.1, Part 1.
"its use is not recommended for descaling sensitized austenitic stainless steels"

 

[mcguire]

Kenvlach
If the stainless grade and the fabrication techniques were chosen correctly, there should not be sensitization.

 

[kenvlach]

mcguire,
True, but I bet you've diagnosed plenty of corroded welds where sensitization was a problem.

By the way, since you are the SS expert, I originally included '304L, 316L, 321, proper filler metals, etc.,'
but I am fighting a tendancy to give wordy answers....

 

[111121]

Kenvlach
Yes, there is a chance, as post weld heat treatment is often written into a specification to avoid problems associated with sensitisation. Sometimes this is impractical due to the size of the vessel, but McGuire is right, if you use correct techniques, and specify materials that have very low carbon content or alloyed with a stabilisating substance such as Ti or Nb, then the risk is minimised. If the material is sensitised, and post-processing annealing is not specified,then an incorrect specification for passivation will be the least of your problems.
We are getting of the point here, UKCats original question related to corrosion potential on a machined flange face, due to pick-up or contamination from the cutting tool. Flanges for pressure vessels are normally made from annealed plate or forgings. The face of the flange is rarely heat affected, especially if they have been manufactured using laser or waterjet. Lubricants or coolants are used in the machining process, and many machine shops and others are not aware of the problems that can result from splashing them all over special alloys, and not correctly cleaning prior to further processing or putting the component into service.
Apart from the particle contamination issue mentioned earlier, lubricants contain a series of chemicals that are not only dangerous to humans, but also highly dangerous to stainless steels. Cutting fluid residue can also affect weld quality if the surface is not cleaned prior to weld deposition. Look at your Material Safety Data Sheets, and ask a corrosion engineer for a opinion.
To answer UKCats question, contamination of the flange surface will rarely come from contact with the cutting tool, it is more likely to be introduced by anything else that touches the surface, from marking pens, other processing operations including welding and grit blasting,through to contact with any specified gasket material or sealant. At least that is my experience down-under.
TJA