Hello, I am seeing inconsistent passivation results in my stainless steel finishing process. I am welding 316 stainless steel, then grinding down the weld to achieve a smooth finish, then electro-cleaning the mechanically altered surface with a handheld electropolisher.
The problem is that sometimes I will get grooves in the ground surface during that step (chatter-like marks perpendicular to the grinding direction) that eventually rust in a salt spray test, even after electrochemical cleaning. I've attached a picture of the marks that show up after the grinding step.
Any ideas on how to avoid rust in this scenario? Is the grinding step thermally degrading the passivation layer of the stainless steel in a way that it cannot be salvaged?
Any feedback is appreciated! Thanks!
Dear Dr. Amy Geary ,
could you attach the picture of the marks that show up after the grinding step? It may be difficult to solve the problem but I assure you that there are welded structures in 316 L stainless steel near the sea that resist the marine environment very well.
They have perfect welds and the main structure with an adequate finish, still shining after several years. Given the size, I don't think they receive particular care... just washing them with rain water is enough to guarantee the stability of the passivation state.
But everything must be very careful, both in production and in installation...
My best regards, Pierluigi Traverso.
Dear Dr. Amy Geary ,
thanks for the photo, I am not an expert in surface finishing systems but I deal with corrosion of metallic materials and I would say - even just by sight - that something is not working correctly...
I recently saw a SS 316 L structure with parts welded and then, most likely, finished with fine grinding. The weld was noticeable but was very smooth and consistent. It integrated very well, even in colour, with the welded pieces, and showed no corrosive attacks and/or signs of inhomogeneity, indicative of work carried out correctly.
In your specific case, personally, I would analyze each production phase to be able to understand when the anomalies that are highlighted with the last surface treatment actually originate.
Good luck and my best regards, Pierluigi Traverso.
Amy
It looks like you processing the right way, but I suspect the stainless material you are welding and grinding, could be the formation of Cr-carbide, depleting chromium content and causing rusting in the salt solution. Try to carry out SEM X-ray dispersive analysis to see any carbide formation. Like to see your results of the analysis.
Dr. K
A number of years ago we had a similar problem and we found information that suggested that the grinding process can fold over the surface and create a crevice. I suggest that you may be getting an inconsistent ground surface before polishing.
It was more to do with manual or powered, type of grinding disc. I will see if I can find the information and send it to you
I'm not sure, but I think this phenomenon is related to welding process, not the finishing.
Porosity is the formation of holes in the weld pool resulting from gas bubbles that cannot escape, appearing after finishing process.
Porosity is usually one of the common welding defects when using shielding gas, which is present in welding techniques such as TIG and stick welding. Absence, lack, or too much shielding gas may lead to metal contamination, which reduces the strength of the weld.
On the other hand, severe versions of porosity come in the form of blow holes or pits when large gas bubbles get trapped in the weld pool. Additionally, smaller gas molecules can blend with the weld metal, forming an impure compound.
Causes
Prevention
I remember somebody telling me about the surface finish effect many years ago. I have checked with an old colleague and he is not sure whether the work was ever published but I understand it came out of a research project in the early 1990's where mechanical grinding produced a coarse finish and in some instances the peaks were folded over the troughs and produced a crevice. He is checking to see if the work was ever published. Will let you know when I know.
I have spoken to a former colleague who remembers the paper but has not got a copy of it in his personal file. He suggests it was probably a Sintef paper from between 1992 and 1996. Unni Steinsmo or Trond Rogne may have been the authors. Sorry I have not been so much help and it has taken a while but maybe you could search for it and be lucky find it in the Library?
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