pH shifts and stainless steel

[rysep]

I was wondering what the effects on large pH shifts are on stainless steel. We have a process in which we switch the pH in a couple of rinse tanks from Sulphamic Acid pH 2.0 to Sodium Hydroxide pH 12.0 and 316SS Bearings being used in the process seem to be affected by it. Does anyone now what the effects are and any suggestions for a stainless grade or other material that could handle this pH shift better?

 

[mcguire]

What temperature and concentration is the sulfamic acid? 316 is not fully resistant to this environment at all temps and concentrations.

 

[rysep]

The temp is at around 40C-45C and the concentration is around 2.5ppm

 

[mcguire]

My data tables show 316 to have a stable passive film under both of those extremes, meaning I would expect no corrosive attack. How are the bearings affected? Are there visible signs of corrosion or is there some other type of damage?

 

[rysep]

The bearings fail sooner. There don't seem to be signs of corrosion but the balls seem to wear sooner. The bearings fail sooner after the pH shift and I was wondering if a large pH shift did actually have an effect.

 

[mcguire]

If there is something going on, it's probably related to friction. Lubricity changes may accompany this pH shift. 316, as other stainless grades, has a tendency to gall, i.e adhesive wear, and perform poorly as an un-lubricated wear component.
Can the bearings be Cr plated? Or since the corrosivity of the media is minimal, maybe try a harder martensitic stainless, e.g 431 or 440A.

 

[rustbuster1]

What is the lubricant in the bearings? Surely these process environments are not expected to be the lubricant. The high pH conditions you state are usually very effective at removing oil-based lubricants. Something is not clear here...

 

[rysep]

There is no lubrication it is an open cage semi-precision bearing and it runs a low rpm and moderately low loads.