Bronze fittings in steel boilers - corrosion?

I ask this question just from curiosity - the boiler on my engine is copper.

If a bronze fitting is screwed into a bush on a steel boiler, below the water level, I would expect that there would be some electrolytic corrosion of the steel in contact with the bronze. There are many steel boilers in existence so I realise that this cannot be a practical problem, I would be interested to understand why, and what evidence exists for this kind of corrosion. Is it simply that the plate thicknesses typically used in construction are sufficient to accept some corrosion over a reasonable lifetime?

I would appreciate any comments.

Paul

When 2 different metals are joined  together, depending on their relative positions in the chemical reactivity series as to what voltage will develope and electrolytic corrosion take place. For small boilers there is the problem of internal examination, where as full size boiler with the tubes out can be well inspected internally. If the inner fire box is also out then a man can get inside the boiler. When a copper boiler is designed there will be no need for a corrosion allowance where as a steel boiler will have one of 0.5 for plate under 1/4" and 0.75 for plate of 1/4" and above. Whilst steel does not weaken at the temperatures used, copper does, so at a working pressure of 60- 100 psi the derating is 0.8 and 110- 150psi 0.7. A safety factor of between 6 and 10,though 8 is thought suffient. Hope this helps.

Figures from "Model Locomotive Boilers" by Martin Evans.

Edited By noel shelley on 04/02/2021 12:49:14