Corrosion of Stainless Steel Anodes

[filterman]

Here is the situation:

Our customer uses analyte cells for electrocoat paint application. The analyte cells facilitate the transfer of acid through the ion exchange resin of the cell. Inside the cell is a 316L stainless steel anode. In this particular application, the stainless steel of one supplier is corroding, while the stainless steel of a second supplier is not experiencing any corrosion. We have sent samples of both anodes out for a compositional analysis and both meet the compositional requirements of 316L Stainless Steel. The stainless steel that is corroding is manufactured in Korea while the non corroding stainless steel is manufactured in South Africa.

According to our current information, there are minimal differences between the two paint tanks. Both tanks are use the same paint; are run at 27-28 C, the tanks are grounded and have similar rectifiers.

The analyte solution is a dilute acetic acid solution. The pH at the start is around 2.7. During the course of a few weeks the pH rose to about 4 and the analyte solution turned red. The 316L anodes weight about 5kg each. In a sixty day period one of the anodes lost 1.3 kg in weight. The anodes with the corrosion are 316L SS Schedule 10 seamless tubes. The anodes with no corrosion are 316L SS Schedule 10 seamed tubes.

Can anyone give me an indication of what is going on here? Why would one corrode and one anode not corrode.

 

[mcguire]

Within a given grade corrosion resistance can vary greatly depending on minor element content and initial surface finish.
corrosion resistance is proportional to the Cr content plus 3 times the Molybdenum plus about 15 times the nitrogen content.
Sulfur has a very negative influence and should be kept below 0.001%.
It sounds like you need to create a tighter specification than simply using ASTM, which is pretty loose.
try getting free advice at stainlesscenter.com

 

[bilge]

mcguire gave you some pretty good advice. 316 is a well recognized excellent candidate for this application.

 

[TVP]

Just to reiterate what mcguire and bilge already mentioned, alloy 316L is obviously capable of providing good results. You didn't mention what the specification you are using. Something like ASTM A 269, which covers both seamless & welded ss tubing for GENERAL SERVICE, allows for the sulfur content to be as high as 0.030 %, which will severely degrade the corrosion resistance. I recommend taking a closer look at the sulfur content, which may require you to use a more stringent specification.