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In the automotive industry, to protect steel which cannot be painted (e.g. the gap or seam between two sheet panels), a product known as weld thru primer is sometimes used.

This is a coating of zinc-rich or copper-rich material which is meant to prevent rusting of the steel, especially at the weld points.

From the reactivity table below, zinc is more reactive than iron, so I can understand how this would protect the iron via sacrificial corrosion. Similar to galvanized steel.

However, copper is less reactive than steel. When salt water get's in between the sheet metal seams, would this not corrode the steel? How would applying a layer of copper to the steel protect it?

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Well there are also many different kinds of steels with different galvanic characteristics are well. For example, all of these metals have a higher reduction potential than copper:

1. Red Brass
2. Stainless steel 347 (active)
3. Molybdenum, Commercial pure
4. Copper-nickel 715
5. Admiralty brass
6. Stainless steel 202 (active)
7. Bronze, Phosphor 534 (B-1)
8. Monel 400
9. Stainless steel 201 (active)
10. Carpenter 20 (active)
11. Stainless steel 321 (active)
12. Stainless steel 316 (active)
13. Stainless steel 309 (active)
14. Stainless steel 17-7PH (passive)
15. Silicone Bronze 655
16. Stainless steel 304 (passive)
17. Stainless steel 301 (passive)
18. Stainless steel 321 (passive)
19. Stainless steel 201 (passive)
20. Stainless steel 286 (passive)
21. Stainless steel 316L (passive)
22. AM355 (active)
23. Stainless steel 202 (passive)
24. Carpenter 20 (passive)
25. AM355 (passive)
26. A286 (passive)
27. Titanium 5A1, 2.5 Sn
28. Titanium 13V, 11Cr, 3Al (annealed)
29. Titanium 6Al, 4V (solution treated and aged)
30. Titanium 6Al, 4V (anneal)
31. Titanium 8Mn
32. Titanium 13V, 11Cr 3Al (solution heat treated and aged)
33. Titanium 75A
34. AM350 (passive)
35. Silver
36. Gold

(source: Corrosion Doctors)

So, as long as you're welding any of those under copper's boiling point of 2,562°C (or 4,644°F, at which point it could just float away from your intended target), then it should be fine for your application.

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