Dissimilar metals

[Parsnip]

I am designing some equipment for use in high salinity conditions. The probelem is that although I would like to use all steel I have a tight constraint on budget. Can I successfully mix aluminium and steel components (if so what protective coatings should I use) and can I use zinc alloy (e.g. BS1004A) for any load bearing parts.
Thanks in advance

 

[Metalguy]

These answers depend on what your environmental conds. are. Besides the high salts (Cl-?) what temp. and flow, etc.

The alum. will tend to corrode as it tries to protect the steel, so you will need a good coating-such as a high-build epoxy, etc.

 

[Parsnip]

The environmental conditions aren't too bad, normal temperature range and a flow of upto 2 m/s. Apart from the salts there maybe particulates of various sizes and materials and to cap it all on entry into the fluid it will be travelling at 30m/s. When in there there will be a pressure of approximately 150 psi which is why I was asking about zinc as I have never used it before.
Cheers

 

[Metalguy]

I suspect that high initial velocity (if I understand it right) will have a very high wear rate on zinc and it's alloys. Can you use stainless steel for the high-wear areas?

Zinc alloys aren't usually used for high-stress applications.

 

[Parsnip]

Thanks for that.
I have thrown zinc out of the window as it were, and am going to be using zintec coated steel sheet and al alloy (possibly 7075 T6)so hope that the corrsoion problem is solved. However, can you think of any other material (preferably cheap) that is free machining that may be better suited to the environment.

 

[TVP]

7075-T6 suffers from stress corrosion cracking in environments with high levels of chloride ions. I would recommend you perform a more thorough examination of the alloys to be used, stress conditions, etc.

 

[Metalguy]

How long do you need this assembly to last? How long will it be wet?

Zinc coated steel won't last too long in aggressive water.

It's hard to envision just what you are building.

 

[ugwuegbuchima]

It is not wise to use dissimilar metals for a design in a high salinity condition(where NaCl is present).Their contact results to galvanic corrosion.In a situation were dissimilar metals are unavoidable(since you are considering your budget),the metal forming the anode should not have small surface area as compared to the cathode.As the corrosion current passes through the anode and the cathode,a small anode area would mean a high current density at the anode and a consequent high rate of corrosion.Since you are to combine Al and steel,both Al&steel are in the active (anode)end of the Galvanic series seawater table,but steel is nearer to the noble(cathode)end than Al,it implies Al will form the anode while steel will form the cathode.Hence while carrying out your design,Al should have high surface area than steel.
For the protective coating to be used,use Zn coating for the design.Zn coating will give the best protection needed.This is because Zn is anodic relative to Al&steel,any break in the coating will not lead to corrosion of the design(Al+steel),which is cathodic and preserved.