Black stains after pickling and passivation. 1

[MVlem]

Hello all,

One of our contractors is constructing a 316 vessel for us. After pickling and passivation i went over for an inspection and saw some black marks on 1 or 2 welds.
The QC-manager told us these were due to over-pickling and will not have any detrimental effect on the vessel.
That last statement i'm not so sure of. I'm not an expert, but according to what i can find these marks must mean there are still some contaminents or oxides left over in the weld and these should be removed.
Or am i completely wrong?

Kind Regards

Marc

 

[cloa]

Whether it really matters probably depends on the service- if its not near a limit for 316 then it probably won't make a difference. Maybe you want to consider your removal options and whether you try to remove quickly and cheaply and not make it worse like add steel contamination which would seriously impact on 316 corrosion resistance.

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[mrfailure]

Tough to answer this one without at least a picture. It may be worth it to you to remove one of the black areas as a boat sample and send to a lab for analysis (and repair welding the material removed).

 

[EdStainless]

It could be residual oxide, or it could be re-deposited residue from the pickle solution.
Or it could be an area that is very rough/porous from over pickling and it looks black because the texture is different.
Most times you would see a fabricator touch these up with a hand held flap wheel and then spot pickle/passivate to 'fix' them.
Visual variations often draw attention, but they are rarely significant in terms of performance.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube