Rusting Bolt Question

[Jake Robinson]

We installed new SS air piping into a aerobic digester and are encountering a rusting issue on some of the 316 SS bolts on the pipe flanges. The bolts have markings on them that either say "316" directly or "G8M" which indicates 316 SS as well. The "G8M" bolts seem to be rusting. (see image). any reason on why this might be? Thanks

 

[GC_Hopi]

I have seen something similar on a pump. That issue was the steel was not fully coated leading to corrosion between dissimilar materials and rust staining. The un-coated areas were the mating surfaces at the flanges and the inside of the bolt holes. I am not sure how it was worked out but recoating was necessary. Also periodic inspection and an extended warranty were recommended.

 

[dik]

You might try an acid wash to passivate the material... also using ferrous tools for installation can promote 'rust'.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bimr]

Assume that this is installed out side of the digester. Are the nuts also stainless; the nuts don't look like stainless.

The bolt on the left has corrosion on it from contact with ferrous metal.

Remediation

Contamination on Stainless steel surfaces with free iron is common. It can be avoided only with very careful handling. The presence of free iron on the surfaces of interest can be detected by a variety of tests, including the copper sulfate and ferroxyl tests. Iron contamination can be removed by certain chemical or electro-chemical methods; abrasive blasting alone is not effective.

Do you have a picture of new bolts? Maybe you can check the source of the bolts to determine if the quality if poor.

It will probably be easier to replace the bolts than try to remove the rust through passivation.

 

[swazimatt]

the rusting/discoloration looks worse on the underside, there also looks like there is a crack in the coating

 

[MotorCity]

Maybe the bolts are not rusting. Could it be rusty water dripping from above and staining the bolt heads? It looks like all of the bolts with rust are on the same side. And there appear to be rust drips on the blue flange below.

 

[dik]

It's been mentioned a few times on this site that white Head and Shoulders Shampoo can detect traces of iron on SS. It works, I checked it myself.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Truds]

Looks like galvanic corrosion between dissimilar metals. Look at using a bolt isolator such as Top Hat or Mylar Sleeve and Epoxy Washer.