Zinc Coatings
It's the New Year and I feel I should be talking about something new and exciting. That's not too easy to find in our rather well worn field! I did introduce something a bit different in my class at the college last week, asking the students whether the term corrosion should be used to describe not just metals deleteriously interacting with the environment but ANY Material. The feeling was they didn't like it. Anyway enough of new stuff! What I want mainly to talk about today is at least as old as the hills. That is galvanizing or more strictly zinc based coatings.

When I am out and about, often walking in hilly areas, I see quite a few examples of the good, the bad and the ugly in terms of coated metal and I sometimes photograph them. As my pictures show, galvanising does sometimes fail! Perhaps we should expect it to? Can a metal with a lower potential actually corrode more slowly than the metal it is trying to protect? (I have a bit of a philosophical problem with this - maybe cleverer people than I (like our President!) can explain this to me one day).

It is generally agreed that zinc forms a protective carbonate/ sulphate but that must surely ennoble it i.e. reduce its tendency to corrode. Maybe it still corrodes faster than the iron would have done and it is just that the corrosion
product being white makes it look like it has not corroded significantly? Anyway this problem of straight zinc corroding too fast has been addressed in recent years by developing alloy coatings (zinc-aluminium, zinc-cobalt, zinc-manganese to name just a few). These normally corrode more slowly than pure zinc. However as their potential gets nearer to that of iron, their effectiveness at protecting iron at a break is reduced. Its swings and roundabouts and the environment will be critical.


There has to be a balance between formation of protective oxide / corrosion product films and reactivity. Intelligent zinc alloys would corrode at just the right rate so that, in any particular environment, they act to just protect the underlying metal. On a related tack I also still get enquiries about galvanizing in relation to difficulty painting over it. Traditional methods of dealing with this were T wash (solution based on phosphoric acid?) or allowing the zinc surface to "weather" before painting. Incidentally the tendency for paint to fall off zinc surfaces is presumably because, once hit by water and oxygen (proof if it were needed that uncharged species get through paints), zinc reacts to form a friable layer and also creates alkali. Creation of a phosphate (or similar layer) presents this.


Anyway, no doubt zinc and related coatings are going to be with us for some time to come. I for one am glad about that as it makes life more interesting and also zinc is very reliable (I can always get exactly 1 Volt (SCE) from it when we do experiments in the lab!) Going back to last month's column which talked about lab (accelerated) testing, someone asked me what a typical acceleration factor might be. Well off the top of my head I did not have an answer but I have since seen a salt spray cabinet in the company where an MPhil student is working where they suggest 1850hrs in the cabinet is equivalent to 2 years (factor of 8) outside. Comments on that (or anything else I've written here) are, as always, welcome douglas@harrbridge.freeserve.co.uk