Thorley
Marine/Ocean
- Mar 31, 2011
- 6
Hello All.
The subject of this topic is corrosion resistant coatings for 4140 base material. The application is a shaft of a linear actuator used in a saltwater spray environment (aft deck of a ship). The current shaft is processed with a conversion coating known as QPQ (quench polish quench), which I understand is a salt bath nitriding process used to develop an oxide layer at the surface of the base material to aide in wear and corrosion resistance.
We have recently observed surface rust on some of the shafts, leading us to believe perhaps this is not the appropriate coating for the saltwater spray environment. I have further researched the QPQ coating, and found data suggesting ASTM B117 corrosion test results yield ~ 500-600 hours prior to the onset of visible rust for this coating on a 4140 substrate.
We are looking to select a different coating for the shafts, however I am unable to find data on what is most suitable. Our goal is to just swap out the coating process, i.e. NOT have to change shaft dimensions to accommodate a thicker, non-conversion coating (there are a lot of shafts machined already which would require further machining if we begin to mess with tolerances).
My question is: What is the suggested coating for my application, and what test results (i.e. ASTM B117) should I look at in order to get an objective measurement of the coating performance with respect to corrosion resistance?
Thank you
The subject of this topic is corrosion resistant coatings for 4140 base material. The application is a shaft of a linear actuator used in a saltwater spray environment (aft deck of a ship). The current shaft is processed with a conversion coating known as QPQ (quench polish quench), which I understand is a salt bath nitriding process used to develop an oxide layer at the surface of the base material to aide in wear and corrosion resistance.
We have recently observed surface rust on some of the shafts, leading us to believe perhaps this is not the appropriate coating for the saltwater spray environment. I have further researched the QPQ coating, and found data suggesting ASTM B117 corrosion test results yield ~ 500-600 hours prior to the onset of visible rust for this coating on a 4140 substrate.
We are looking to select a different coating for the shafts, however I am unable to find data on what is most suitable. Our goal is to just swap out the coating process, i.e. NOT have to change shaft dimensions to accommodate a thicker, non-conversion coating (there are a lot of shafts machined already which would require further machining if we begin to mess with tolerances).
My question is: What is the suggested coating for my application, and what test results (i.e. ASTM B117) should I look at in order to get an objective measurement of the coating performance with respect to corrosion resistance?
Thank you