Passivating MOD420SS. 1

[NickE]

Is passivation effective on high tensile strength, (>270ksi), quenched and tempered modified 420ss?

Our customer is using the material as as flexure strap on a power washer, the assembly is the SS strap clamped btw a galvanized plate, and a 380 Al casting. eventually the crevices begin to show some corrosion ruining the highly prepared surface and causing fatigue failure initiation and subsequent failure.

Chemistry of MOD420SS:

C: 0.36-0.40%
Si: 0.30-0.50%
Mn: 0.45-0.65%
P: MAX 0.025%
S: MAX 0.010%
Cr: 13.2-13.8%
Mo: 0.90-1.10%

This is a special SS melted in small heats and with extremely low inclusion counts. We stamp the parts from ~HRc 50 coil and then do some edge/surface finishing in order to increse the fatigue life. Our custome asked if passivation would help prevent the corrosion from starting.

Will it?

MY thoughts: redesign or change materials since this steel is barely stainless.

nick

 

[kenvlach]

Passivation will help, but may only delay the inevitable. Try it first, as it is less expensive than redesigning.

Federal spec. QQ-P-35C shows a Type II passivation for 420 SS. This consists of 20+ minutes in a 2.5 wt% sodium dichromate dihydrate + 20-25 vol.% nitric acid (67 wt% nitric acid basis) at 120-130[sup]o[/sup]F. Note: QQ-P-35C is obsolete but still frequently used, and is free at

http://assist1.daps.dla.mil/quicksearch/

Stellar Solutions has versions of their citric acid-based product suitable for martensitic SS passivation per ASTM A967. Lee Kremer is a helpful guy:

http://www.stellarsolutions.net/products/prodline.htm

In a redesign, perhaps chromate & powdercoat both plate & casting.