incoloy 840 & 316 s/s which has better corrosion resistance?

[Marcobs]

I am trying to source submersible elements for catering water boilers. I wanted incoloy 800 but am finding difficulty in getting them at a good price. I have been offered incoloy 840 or 316s/s. All I can find about 840 is that it is an incoloy alloy suited to welding so has it poorer properties than normal 800?

Which has better corrosion properties, 316 or 840?
is there any advantage to the one with poorer resistance properties?

 

[metengr]

One of the web sites that I saw on immersion heaters listed Incoloy 840 tube sheathing as their primary product for tubular heaters. Their list included applications for catering and industrial water heating.

The Incoloy 840 will give you an advatage of higher sheathing temperature in comparison to the 316 ss for quicker heat up (higher watt density). The Incoloy 840 should have better corrosion resistance than the 316 ss.

 

[YLWEA]

I am not familiar with Inco 840, but I would have concerns about possible stress corrosion cracking of a very hot 300 series stainless element immersed in water, particularly if there is a mechanism for scale formation or other concentration of Cl- at the element surface.

 

[Metalguy]

There is little doubt that Alloy 800 is FAR more resistant to SCC in demin. water at high temp. (300 deg C) than 316/316L, and 840 is essentially a lower C, lower Ni version of 800. Don't know about lower temps., but I recall reading about a caution wrt SCC vs Ni content. There is a certain Ni range that is real bad, and I suspect it may be just under what 840 has. I'll have to look it up.

 

[Marcobs]

Thanks for the info. The highest temp I am going to see is 121C in a pressure boiler at 15psi. Scale is an issue in our boilers. I have read that incoloy allows for a shorter length of heating tube but this will lead to more concentrated scale per unit length. In the past we have used copper elements with a nickel plating or chrome plating. In some places the chrome just fell off in a few days. With nickel we experienced a green growth which could occur in a matter of hours. I found out this may be "bronze disease" though are manufacturer denied it was this. Our boilers have probes so the element will always be immersed in water. Some people do use acids for cleaning/descaling our boilers, usually phosphoric but also hydrochloric.