A CORROSION CONTROL TEST FOR STEEL By Don E. Avery (EAA 56426>

45-437 Akimala Street Kaneohe, Hawaii 96744 Metal corrosion is a problem anywhere there is moisture. In Hawaii the problem is much more severe than most places. Sea water in the tropics contains more salt and is warmer than in temperate zones, making it more active chemically. The trade winds carry salt spray from the surf inland causing unprotected metal to corrode even indoors. In order to find a way of protecting the steel parts better in the COOT A amphibian I am building, I decided to make a few corrosion tests. Fifteen years of experience with my 43 foot homebuilt yawl in tropical waters was enough to demonstrate the importance of protecting metal. Even hot dipped galvanized steel will last only a few years. If the 4130 steel parts of my COOT are to last, they must be well protected. THE TEST — Figure 1 shows a piece of 4130 steel before and Figure 2 shows the piece after 7 months of exposure to sea water. The exposure was accomplished by tossing the piece on the ground and pouring sea water on it 3 or 4 times a week. It was then hung on a nail using the opposite end each time.

20 SEPTEMBER 1973

Two kinds of paint were used: DEVCON Z, manufactured by Devcon Corporation, which is 95% zinc and 5% epoxy binder, and Sears Marine 2-part Epoxy Paint — bright red in this case. There are 4 different areas on the test piece. At the small end two coats of DEVCON Z are covered with two coats of Epoxy. Next there are two coats of DEVCON Z with nothing over it. Next is an area of bare metal and on the big end are two coats of the Epoxy paint. Before painting, the piece showed signs of beginning to rust so it was well sanded before painting. The piece looked bright, but sand blasting and/or chemical cleaning would have done a better job. After the 7 month test, Figure 2 shows that the bare metal rusted badly as expected. The Epoxy at the big end is blistering in spots as rust is lifting up the paint. The DEVCON Z is also blistering in places, but not in others, indicating that a better cleaning job would have helped. The zinc in this paint gives protection by galvanic action in addition to the protection it gives by sealing out moisture. The big surprise is the area with the Epxoy over the DEVCON Z. This part is bright and shiny just like it was at the start, without a single blister. Only ,. the sharp corner where the coatings were either knocked off or were not put on well is there any sign of corrosion. This combination obviously is excellent for protecting steel against rust.