Sudden failure of pump shaft made from 416 SS

[waynebgordon]

Hi
I have a failure analysis of a pump shaft made from 416 SS. It survived for a very long time (> 15 years), then suddenly showed signs of corroding. It has been removed from service, and is highly pitted. Corrosion product inside pits shows small amounts of Cl, Ni, S and high levels of Mn. Also traces of alkali and alkaline earth metals, and vanadium. The Cr and Fe contents of the oxide are about equal.
Does anyone have any explanation as to the the sudden pitting?

 

[mk3223]

How you know it's the "sudden pitting" and when it was happen?

Did you verify the actual pump shaft material and/or any process change for the pump operation recently?

 

[gtaw]

416 is a martensitic, free machining stainless steel. It doesn't offer the same corrosion resistance as austenitic stainless steel. However, corrosion resistance is a relative term. It all depends on the type of corrosion and what is causing the corrosion.

Did the nature of the service change?

Best regards - Al

 

[EdStainless]

The machine shop may like 214, but it has markedly lower corrosion resistance than 410.
How it was heat treated matters also. Was the shaft Q&T?

It doesn't take much Chlorides, especially if the pH was below 7 for pitting to happen rapidly in this alloy.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Typical MnS inclusion cause. High S in this free machine grade is the culprit. The high dissolution rate of MnS inclusions resulted in the formation of a “sulfur crust” over the inclusion that concentrated chloride underneath it by electromigration.
This was probably a gradual process: fatigue initial, critical crack propagate, fracture.

 

[EdStainless]

yes Ben, I forget that most people don't know that free machining grades of SS have crap for pitting resistance and even worse fatigue properties.
The same inclusions that help machining hurt just about everything else.

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P.E. Metallurgy, Plymouth Tube

 

A proper failure analysis is in order here.

This is the kind of case where learning the full service history (which means the history of the process conditions) will be the key. It is more important than the obvious metallurgical evidence, which sounds pretty straightforward, and makes the difference between a root cause failure analysis and a report that just describes the failure mechanism.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."