This week I needed to use some screws for an outdoor project. Initially I chose to use door hinge screws because I had a bunch of them on hand. After I thought about it some, I decided to purchase self-tapping zinc-coated screws under the impression that these would not be as susceptible to corrosion. After I made the purchase I thought about it even more. Then I decided to do some sniffing around online for a definitive answer–which is better? I could not find any simple answers. Apparently corrosion resistance is entirely application- and environment-based.
I did find some useful pointers. The biggest concern mentioned in many sites seems to be galvanic corrosion, where two differing metals are in contact with each other. Each metal will have a unique electronic signature; therefore, one will behave as an anode and another as a cathode. The clincher is that this contact would be made in an electrolyte solution. For example fastening a copper plate with a steel screw in the presence of liquid fertilizers will cause galvanic corrosion quickly. Salt water will produce similar results. Rain water–this is fairly weak–will take a much longer time. This may be mitigated or by selecting metals with similar electrical properties or avoiding the use of one metal altogether. Fortunately my application is a metal screw used to fasten PVC–not another metal.
Here are just a couple of useful facts concerning the two metals I mentioned in the title. Stainless steel is an alloy composed of at least 10.5% chromium. It is very prone to corrosion from certain chemicals–the chloride ion, hydrochloric acid, and sulfuric acid. Alternatively, zinc coating reacts with the environment to form a protective shell that turns the metal into a dingy gray color. If the shell is compromised, then corrosion takes place. When choosing metal alloys for fasteners, think about both the application and environment, then do some research.