Coating of steel in a CP ennironment

[SteveyB]

Hi All,

I was wondering what the opinion was on subsea coatings for structural steel e.g S275/S355.

The company I work for has used many different coating such as, thermally sprayed aluminium, epoxy coatings, diffused nickel plate. The thing is most of these are not suitable for mechanical moving components such as pins etc where a good surface finish is required. So I was wondering what other people use for such situations for long term subsea service?

 

[SJones]

Are you asking about structural steel, about dynamic mechanical components, or both?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[SteveyB]

I am asking about coatings for dynamic steel components.

 

[SJones]

Coatings on the static parts serve three purposes:

1. Corrosion protection
2. To reduce the number of sacrificial anodes
3. To facilitate a more uniform distribution of CP current

Since 'pins' will be relatively small, in the scheme of things, there is, generally, no sharp intake of breath when they are selected in corrosion resistant alloys that require neither coating, nor CP, for corrosion protection. The alloy could be either solid, or weld deposited on a substrate of cheaper, less corrosion resistant material. It may also be that the 'pins' require hard facing to control wear.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[SteveyB]

Thanks for your reply. Sorry my question may of been a bit mis-leading. I was relating to mechanical components in general large or small. Basically I was after coating recommendations for steel components that require a good surface finish to function. Where as component that are just dead steel with just a structural function for example can be coated in TSA and epoxy paint coatings.where are rarely suitable for component with a mechanical function.

 

[NickClark]

For long term subsea service you will no doubt have cathodic protection. In this case and due to the function you may have to leave the surfaces bare. As Steve suggests you can consider stainless steels/CRA's if the parts are shielded or isolated from the cp System. Note that bare steel and bare stainless are treated the same in cp calcs so there may not be an advantage to use stainless. For close tolerance items with some movement or contact dry film lubricants may be suitable such as xylan Or MoS2