Mystery pitting corrosion on 316 parts

[OSU Engineer]

Long story short, we ran several dozen parts from 316SS and sent them off for DLC plating. The plater sent a handful of them back saying they were damaged. Upon inspection, the parts were covered in what appears to be a salt or calcium type powder and after cleaning it off there is pitting into the material - see attached pictures. Also, the parts that have this pitting all have a gold tint to them, whereas the other parts have the normal silver stainless appearance. The pitting seems to have spread in an organic fashion and not from spray or spillage.

We cannot figure out how this happened so that we can prevent it in the future. Has anyone experienced anything like this? Is it possible the plater pre-cleaned them with chlorides or something, and if so why did it only affect a handful of them and why do those parts also have a gold hue.

 

[mfgenggear]

Op

So the parts were manufactured in house.
What type of plating?
Was there any type etching or coating in house.
It appears the plater did some thing.
And is not taking blame.

 

[OSU Engineer]

Op

So the parts were manufactured in house.
What type of plating?
Was there any type etching or coating in house.
It appears the plater did some thing.
And is not taking blame.

Manufactured in-house, sent out for grinding, and then our inspection department spot checked a few that were good and sent them off to DLC plating. It's possible inspection missed the corrosion since it only affected a handful of the parts and they only spot checked a couple. Plater claims they received them this way.

 

[mfgenggear]

Op
Have them send a copy of their process planing for review.
Did they take pictures After opening the container.

Were the parts cleaned and passivated after machining

 

[mfgenggear]

Op
By visual is appears parts were pasivated baked or pre baked, for plating. It is common to bake parts over 40 HRc for hydrogen embrittlement

 

[mfgenggear]

If parts are left to long in a pickling solution it will be attacked. Causing IGA, pitting and more

 

[EdStainless]

But the residue is likely from the grinding operation.
Before the DLC they should be degreased, acid passivated, and rinsed (preferably in warm DI water).

 

[mfgenggear]

Ed ground parts are mirror finish and shine.

 

[mfgenggear]

These appear to be in acid solution and not neutralize properly

 

[EdStainless]

So what if they are shinny.
Are they clean?
Part of this could be related to material that has been smeared in the grinding/polishing process.
Not only does this form crevices but impurities get trapped underneath.

 

[mfgenggear]

Acknowledge

 

[mfgenggear]

Op
Send two of the worst parts to a met lab to obtain analysis. To solve what the contamination is. Discuss with the met lab options for analysis.

 

[WKTaylor]

FYI.. related...

NASA Deals with lots of SSTL issues in their various seacoast environments: NASA TN D-6519 Corrosion Study of Bare and Coated Stainless Steel.

I suspect alloy or residual surface impurities [machining lubricants with chlorides, etc]... not fully removed by the pre-cleaning and subsequent passivation processes.

/NOTE/ I have an annoying hot airduct assembled from multiple fitted pieces of 321/347 SStl sheets. Oddly in the 'same operating environment'... one particular formed sheet section seem highly prone to pitting... others 'not-so'. Still scratching my head over this one, too.

 

[mfgenggear]

So an audit of the process planning and all the operation is required. Operators and inspector stamps and which machines were used. The cutting fluids and any process used.eg heat treat, passivation, liquid penetrant, stress relief. Etc, until the source can be found.
I too in the past delt with stainless steel parts with pitting and contamination. Once parts have corrosion its almost impossible to correct.

 

[EdStainless]

One of my favorites, no paint markers allowed.