STAINLESS STEEL FOR ENVIRONMENTAL CORRISIVE ENVIROMENT

[JSZA]

As an Electrical Engineer the following metallurgical problem is outside of my expertise. Normally we request the use of brass valves on large power transformers. The main reason for this is corrosion resistance of the brass components. One of our transformer suppliers now offered a stainless steel valve because of difficulties obtaining brass valves. To my knowledge not all stainless steels are corrosion resistant. The particular environment where the unit will be installed is very corrosive because of high ambient temperatures, high humidity and high marine pollution (Middle East close to the coast). The supplier provided the following material specification for the valves: X5 CR NI 18 10 DIN EN 10088-2. Can anybody offer any comment on the suitability of this quality of stainless steel valves for this environment. We would expect a useful life span equivalent to the transformer i.e. 30 years or more.

 

[TVP]

My opinion is that this is not suitable. If there is no molybdenum in the alloy, it is not suitable for coastal environments.

 

[strider6]

the material suggested by your supplier is a SS304 stainless steel. considering the evironment (marine env) it's not the best solution.. you should consider the operating temperature for possibility of scc (chloride stress corrosion) and also pitting at lower temperature. one solution is to upgrade to a SS316 which has higher resistance to pitting but not to scc. the other is to paint the valve with a suitable coating and to inspect the valve regurarly. if the design life is 30ys i'll prefere the SS316 painted...
this is for external corrosion.. but.. what's the fluid inside the valve??

S

Corrosion Protection & Corrosion Control

 

[EngAbdelkader]

JSZA,

The material you are speaking about X5 Cr Ni 18 10 is what is commonly known as AISI 304!

it is an austenitic stainless steel and it is sensitive to corrosion by Chloride where it suffers from pitting and also what we call stress corrosion cracking (SCC) so care should be taken when using this type of materials.

In regard to the environment you described I would say that a copper-nickel alloy would be a good choice and a good balance between quality/price in comparison with a duplex stainless steel that can also handle your problem!

anyway don't forget that the choice of a good material is not s sufficient solution for corrosion problems, in addition you should think to provide your plant with a corrosion inhibitor injection unit with the right inhibitor obviously!

 

[EngAbdelkader]

here is a list of some materials and their resistance to different corrosive media (curtsy of engineering toolbox)I've just added some colors!

hope it'll help

kader

 

[Teabag12]

Just to say that when we've used 304 it has rusted before it's left the factory. For offshore work we will only use 316L

 

[davefitz]

A useful plot of pren vs critical crevice temp for all corrosion resistnat materials is as per Janikowski (2003), see the first figure in this link:

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http://www.plymouth.com/_tmp/SelTubingMatforPwrGenSWChem.pdf>

This alloy chart is conservative and meant for heat exchanger service; bolts and fasteners might be selected in a less conservative manner. But it seems clear that the 6% Mo alloys are clear winners in seawater service, and the 300 series stainless steels will have problems

 

[strider6]

try also this:
Stainless Steel Selection Software

The Stainless Steel Evaluation System is designed for use by architects, contractors, and others involved in stainless steel selection. It is based on practical experience and atmospheric exposure studies.

http://www.imoa3.webinfoserver.com/Default.asp?Page=96

S

Corrosion Protection & Corrosion Control

 

[joeswoes]

I'm surprised no one guided you to the obvious, non-metallics. What is the design/pressure requirements of your valves ?

 

[JSZA]

Dear All,

This is the first time I used the forum and I have to thank you all for the very informative responses.

The transformer is filled with approximately 25,000 litres of electrical insulating oil. The oil is circulated at low pressure through a radiator for cooling purposes. The valves are mostly located between the main tank and the radiator and between the main tank and conservator tank (gravity feed) and a few valves are used for external sampling points. Therefore no requirement exist for high pressure or high flow rates.