Critical Pitting Temperature (CPT) 1

[mead]

Hello all,

I have some questions relating to how to interpret CPT data.

1. Can the CPT be viewed as a maximum value to not go beyond? If you stay below the CPT, no pitting? If you go above does breakdown automatically occur?

2. I know that CPT test are done with a 6% ferric chloride solution or a 1M concentration NaCl solution. For practical purposes if your concentration of NaCl is above or below 1M would the value of the CPT change?

3. What in plant practical use are CPT values if they are based on standard tests that your process will not duplicate?

4. For a given concentration of NaCl in stainless steel (304L and 316L) will lowering the temperature reduce the potential for corrosion? Is there a point below which lowering it any further will have no effect?

What I am trying to determine is what temperature we should store our products at in 304L and 316L stainless tanks given a known NaCl concentration and if it is worth spending money on.

I know this was long, but I will aprreciate any thoughts, suggestions, bumps in the right direction, or references.

Thanks

 

[mcguire]

1.The idea of a critical pitting temperature is to have a temperature below which pitting will not occur in a standardized environment.
2.The temperature at which pitting will occur will change with environment.
3.The CPT is more a standard by which to rate materials than it is a predictor of performance in an environment other than the standard one. There is some correlation that can be made and I put it in an FAQ in this forum.
4. If you looked at the FAQ it gives the influence of temperature. It has a very strong effect.

Stainless should be chosen such that the chance of pitting corrosion in foreseeable environments is nil. It is usually knowledge of the environment that limits this selection's accuracy. The other FAQ tells how to improve the pitting resistance of the alloy itself to make it fit the environment. I hope there's enough information there for you.