Effects of hydrochloric acid on Galvanized coatings?

A. I would certainly be looking at switching to a stainless steel cable. Preferably a 316L (the L is for low carbon which makes a difference) material. There are some marine cables with more than the normal 18-8 chrome-nickel of 304 types. If you could find something like a 20-10, it probably would last as long as the 316SS which has Mo in it. In answer to your question, there is no answer. All lab or theoretical rates are under a given set of circumstances, which you certainly do not have.

A. HCl reacts furiously with Zinc (Galvanize), and reacts rapidly with steel (pickling process). Even when neutralized, the Cl is still reactive towards both Zinc & Steel, and will, in a short period of time, completely remove the Zinc and start rusting the steel if not copiously rinsed and dried, and even re-chromated. Big hassle in the field. As for Stainless Steel cable, even 316L is not recommended for wet Chloride service. You must use a Molybdenum stainless such as 317, or other such Moly containing alloy such as Haynes C276. They are not as strong but will withstand the wet chloride environment.

Another option is Tin plated cable instead of Zinc. Tin holds up much better than Zinc in chloride. Platers know that you strip Tin in Fluoboric/peroxide mix; HCl won't do that job. But as the cable is used It will get scratched & abraded, so any coating will eventually give out and put whatever depends on that cable at risk. Therefore the correct grade of material should be selected for the intended service.

A. Hi Troy,

Given your problems, why don't you consider some plastic rope?

DYNEX of New Zealand infer that the breaking strain of their cable is better than mild steel. I initially thought of TERYLENE which yachts use instead, apparently, of nylon. Less stretch? Better U.V.?

Personally I have absolutely no experience with plastic ropes BUT if you can obtain one with the right strength and properties, then most or all of them should be OK for the mild hydrochloric acid such as you encounter.

Where to buy them? Try a major ships chandler or better still the INTERNET.

A. Hi Johur. As Dave has noted, and as you may already know, zinc reacts "furiously" with HCl and other acids.

Here's the explanation of how galvanizing can used as a corrosion preventative even though it is more reactive than steel: in normal atmospheric conditions the reaction products of steel and acid are a loose hydroscopic powdery "rust" that retains water and does nothing towards stopping the continued reaction of the steel with the atmosphere, whereas the corrosion products of zinc (under ideal conditions) can form a tough, glassy, impervious barrier that tends to prevent further corrosion. So, in normal atmospheres zinc coatings like galvanizing can be a good corrosion fighter. But in an acidic environment, galvanizing is much worse than plain steel.

Regards,