What might have caused this

[penguegio]

Hi All,

This is a detail picture of a corrosion spot area on a cylinder's rod.
The cylinder is working in a marine environment.

I would like to know from any experience person, what they think it might
have caused this.

Thanks,

 

[swall]

I am assuming that the rod is hard chrome plated. In such case, exposure to salt water caused corrosion to penetrate to the base metal due to cracks in the chrome and as the corrosion spread, it lead to flaking of the chrome. Once the chrome is gone in a spot, corrosion of the base metal continues.

 

[penguegio]

Hi,

Yes, it is AISI 630 with hart chrome treatment.

Thanks

 

[EdStainless]

Hard Cr will not stand up in seawater.
as Swall said, there are enough cracks and porosity in it to allow the under lying metal to corrode.
In fact once corrosion starts the Cr makes it worse since it is more corrosion resistant it will force the steel to corrode faster.

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Plymouth Tube

 

[penguegio]

Thanks,

So which material is the most indicated for a cylinder's rod in a marine environment?

Thanks

 

[swall]

I can't speak to materials commonly spec'd for piston rods in marine application. However, it is possible to improve upon the corrosion resistance of the chrome plating. There is "crack free chrome" which will reduce the tendency of salt water reaching the base metal. You also have the option of using a nickel layer under the chrome. You need to talk to your plating vendor about these options.

 

[TVP]

Proper material selection will depend upon the applied stresses in addition to the environment. The best corrosion performance for commercially available piston rods will be duplex stainless steel like X2CrNiMoN22-5-3 (1.4462), also known as UNS S32205 or just 2205 for short. But is does not have very good wear resistance. Type 630 (17-4 PH) with hard chromium is commonly used for marine environments, but if the conditions are severe, it will not have very good corrosion resistance. Here is a link with some additional information:

http://www.nurmi.fi/files/marine_and_offshore_EN.pdf

 

[boo1]

For our accumulator shafts, and shaft seal surfaces, in the marine enviroment we use Metco TiCarbide HVOF coatings

 

[penguegio]

Thanks to everybody,

The cylinder I'm investigating on has a rod made from 17-4PH (Nikrom brand). We had inquired the manufacturer of these rods and they came up with the theory of galvanic corrosion after we supplied the detailed drawing of the cylinder. They said that, because of an internal bearing made from bronze and because of the salty environment a galvanic corrosion has taken place between the bronze and the chrome plated rod. So basically they claim is the cylinder's manufacture design fault and not a material quality problem.

If anybody has any idea about this galvanic corrosion I will really appreciate to read their point of view.

thanks

 

[boo1]

Alloys nearer the top of the Galvanic Series as shown in this chart are less noble than those towards the bottom. It is seen that bronze base alloys are in the middle of the Series whereas active chromes are appreciably less noble and would be expected to corrode preferentially when coupled to them in a system.