stainless and dzr

[MM5AHO]

Someone asked me about having 316 stainless tubing after a Brass DZR seacock. (hoses after that)
I think that mixing metals in contact and when both in contact with seawater is to be avoided.
What's your opinion on these two?

 

[neil_s]

I have bronze skin fittings with s/s seacocks. Been good for over 10 years.

 

[Kelpie]

I think the theory is that any bronze is anodic to stainless and it will corrode. However I'm not sure how this works in practise and whether being DZR vs another alloy makes a difference. Hopefully Vyv will be along shortly...

 

[VicS]

I think the theory is that any bronze is anodic to stainless and it will corrode. However I'm not sure how this works in practise and whether being DZR vs another alloy makes a difference. Hopefully Vyv will be along shortly...

Yes some wise words from Vyv would be appreciated,

As I understand it stainless steels can behave unexpectedly in electrochemical/ galvanic situations

When it becomes the cathode in a cell as it does with zinc anodes and also with bronze or brass it can lose its protective oxide film and hence its "passiveness". You will notice in many galvanic series charts there are two potential ranges shown for stainless steels . In the one illustrated below the unshaded boxes are the electrode potentials for the alloys in their active states . However you can see they have now shifted to the anodic side of brasses and bronzes. In this position they will now become passivated again as this is chemically speaking now an oxidising environment.

There must be some sort of equilibrium position established in which there is little or no overall interaction.

With zinc anodes you just end up protecting stainless steels as though they were ordinary mild or low alloy steels with no passivity of their own

It is the difference between the passive and active states that is the underlying cause of crevice corrosion.

Shut up Vic you are getting out of your depth !

Perhaps Vyv can explain better

 

[vyv_cox]

I cannot offer any more theoretical explanation than Vic's, I agree with him that the potential of stainless steel will vary between the two points shown in the galvanic series in seawater chart. Further confusion occurs when the stainless steel part is screwed or pressed into the brass, as it might be with a seacock and hose fitting or tubing, where crevice corrosion could be an additional issue.

However, this is very little different where a propeller made of manganese bronze, which is only a slightly modified 60/40 brass, is attached to a stainless steel prop shaft. Although in many cases an anode would protect the galvanic combination there are thousands of examples in which no anode is fitted. I have one of them on my motorsailer and there is no visible corrosion of either the propeller or the shaft.

 

[MM5AHO]

It turns out that the pipe stub that will fit into the DZR ballvalve (seacock) proposed to be in 316SS, has previously been in copper - standard plumbers tubing. The skin fitting that the ballvalve attaches to on the hull side is DZR, and the boat has no anodes (Zn or otherwise) anywhere, but has not suffered significant corrosion in the past. The service is below waterline and is a cockpit drain. The pipe attaches to the ballvalve using a standard plumbers olive, and I'm guessing on this but that olive is probably brass, but not normally in contact with the seawater, being on the outside of the tubing.

So my question is now, why change from copper if that has lasted some decades?

Thanks for the comments.

 

[vyv_cox]

Copper does pretty well in good seawater but can corrode slowly in stagnant water, especially brackish or fresh with agricultural runoff. However most calorifiers have copper coils and even raw water cooled ones last well. Mine was at least 15 years old and still good when I replaced the calorifier.