Stainless steel Carbon contaimnation. 1

[Jamjar23]

We have have a number of stainless steel lines to install but have found carbon contaimnation adjacent to the weld area, it seems like carbon brushers have been used. Can any one advise a suitable cleaning method bearing in mide the site has strict rules on chemicals being used and pickiling is not advisable as the pipe work is installed.

 

[JimCasey]

By "Carbon contamination" you mean "Contamination with trace amounts of carbon steel"
The tiny amount of carbon steel was eroded off the tips of the wires in the wire-brush when the welder cleaned the flux and weld spatter from around the joint.

Your welder has violated procedure by using the wrong tool to clean the joint. That makes all his work suspect. Chemical cleaning is the only way to remove the traces of carbon steel. You said that pickling is not acceptable. I spoke with one of my engineers and he described passivation. There is a procedure in which the part is soaked in a boiling nitric acid solution, or another procedure in which the part is soaked in a warm solution of primarily citric acid.

I only see three coices:
1. You can convince the customer to accept the welds with surface contamination,
2. You can passivate the welds and demonstrate that they have become chemically compliant,
3. or you can scrap all the contaminated piping and start over.

 

[israelkk]

You can use low pressure high mesh (>180) aluminum oxide grit (or glass beads) blast to mechanically clean the contaminated area.

 

[JimCasey]

Israelkk has an interesting point.
Abrasive blasting to clean a contaminated surface must remove the contaminated layer. Otherwise it it just as likely to drive the contamination particles deeper into the surface irregularities, and to peen the parent metal over the contaminants.
These are stainless lines so they are subject to work hardening.
Surface blasting results in peening of the surface and leaves the surface layer in compression. Cracks are thus less likely to form. But the adjacent nonhardened surface will form a galvanic cell and if corrosion is possible it will appear at this junction-much like the tip and head of a nail will preferentially rust since they have been cold-worked.

This might be a good time to have a conference with the design engineer and the code experts in your facility and in your juristiction. Your final choice is as likely to be dictated by code restrictions as by pragmatic considerations.

 

[moltenmetal]

Bead blasting and shot peening are worlds apart, aren't they?!

 

[MartinSr00]

Look at ASTM A380 for information on pickling and passivating.

I know in our shop, we sometimes garnet blast the surface to remove it. If it's a machined part, that can cause problems.

Do not of course use steel grit to blast the part!