Summary for saline-resistant coating Discussion
saline-resistant coating for stainless steel shaft is the core issue here: Colin Bryant described a polished 316 stainless rod that slowly moves and rotates through two retained nitrile O-rings while immersed in 0.9% saline, and after 3 to 6 months the rod surface becomes rough enough to damage the seals and let saline leak past. Toby Padfield said 316 can be plated with nickel or chromium, or diffusion hardened with a process like Kolsterising, but he questioned whether wear alone is the real cause and noted that 316 is not immune to chloride pitting; he suggested higher-alloy stainless such as 317, 904L, or duplex 2205/2507. Guillermo Marrufo briefly recommended high-phosphorus electroless nickel. Tom Gallant suggested softer seals, and Ken Katakia proposed thermal plasma spray chrome oxide followed by grinding and lapping. Bruce Bishkin advised moving to a 6-moly stainless if temperature stays below 180F.
2004
Q. My company is experiencing a problem using 316 stainless steel. A
3.6mm dia. rod passes through 2 retained nitrile ‘o’ rings. In operation, the rod moves up and down through the seals at about
2mm/sec. i.e. very slowly. As the rod moves up and down, it also rotates at a low rate in a spiral manner. The rod is moved up and down by a screw thread. The rod is very polished spec 316 stainless steel. The rod is immersed in saline solution ( 0.9% ) as it moves up and down, i.e. it is always immersed.
The problem is this: Over a period of between 3 and 6 months the rod surface becomes corroded but I suspect that eroded would be a better word. The sharp edges produced by ‘erosion’ chew up the ‘o’ rings and then saline passes through the seals and contaminates everything underneath.
There are no detectable stray volts about so electrochemical action seems a no no. There is another 316 stainless item in the saline bath but not in contact with the rod. In my mind the problem appears to be that the rod surface is not hard enough and can easily pit, leading to corrosion.
My request is this. Is there a hard corrosion resisting metal that can be applied to the 316 but still enable us to maintain a tolerance on the rod diameter. Electroless nickel perhaps?
Thanking you,
Colin Bryant
optical equipment manufacturer – Worcester, Worcestershire, UK
First of three simultaneous responses —
A. Stainless steel can be plated by a variety of metals, including nickel (both electroless and electroplating processes) and chromium. Another option would be a diffusion hardening process like Kolsterising from Bodycote. However, I am not sure that erosion (lack of hardness and wear resistance) is the problem. Type 316 is not impervious to corrosion in chloride environments, so depending on the actual concentration, you may need something with better resistance to pitting and crevice corrosion. Austenitic stainless steels like Type 317 and 904L have better resistance, as doe duplex alloys like
2205 and 2507. You can find more information on these alloys from producers like Sandvik, Carpenter, and Outokumpu.
Toby Padfield
Automotive module supplier – Michigan
2004
Second of three simultaneous responses —
A. Yes. High phosphorous only, no-bake.
Guillermo Marrufo
Monterrey, NL, Mexico
2004
Third of three simultaneous responses —
A. Try softer seals, either O-rings or lip seals.
Tom Gallant
– Long Beach, California, USA
2004
A. Hello,
You should try Thermal Plasma Spray of Chrome Oxide, 0.005″ thick and then grind it and lap it to lowest Ra value. May be 2 or 3 Ra.
Ken Katakia
– Georgetown, Ontario, Canada
2004
A. You should go to a 6 moly stainless. If your temperature is less than 180F it will not pit.
Bruce Bishkin
– Elmhurst, Illinois, USA
2005