Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2003

Protecting Steel Tubes with Zinc of Galvanizing

Q. The Space Shuttle segments are shipped from northern Utah by rail to Florida where they are assembled into the booster rockets. On the railcars, the segments are covered with a fiberglass cover that has imbedded zinc plated steel tubing. If the cover gets damaged and the fiberglass has to be replaced over the tube, the zinc is probably ground off during the grinding of the fiberglass in preparation for new layers of fiberglass. Would a cold galvanizing substance, a zinc chromate primer, or a zinc loaded primer be the best coating considering corrosion protection vs preparation for a good surface bond of the fiberglass resin on the bare steel prior to adding the fiberglass layers? Thanks!!!!

A. What exactly do you mean by cold galvanizing? I understand cold galvanizing to mean a zinc-rich primer, which is the same as what you proposed in your third option.

Here are my thoughts: Provided that you can get good adhesion between the coated steel tubes and the fiberglass, the zinc chromate primer probably offers the least corrosion protection in highly humid and/or marine environments. In relatively low humidity and/or non corrosive environments corrosion should not be a big problem, in which case zinc chromate is probably more than adequate. The advantage of a zinc chromate primer is that in many cases you do not need to abrasive blast the steel tubes down to bare metal, but can thoroughly clean the metal before priming. In the ideal case, you would first apply an iron or zinc inorganic phosphate to give you good adhesion and corrosion protection. Thereafter you can apply the zinc chromate primer.

Zinc-rich primers are available in different resin types, but the key to success is direct contact between the metallic zinc pigments and the bare metal. Therefore, regardless of the resin type, you should abrasive blast the steel tubes to bare metal so that the zinc pigments in the primer can come into direct contact with the substrate and offer the substrate cathodic protection.

Inorganic

zinc-rich primers probably offer the greatest corrosion protection, but the primer must be applied by painters who really know what they are doing. These primers are not easy to apply and tend to be somewhat powdery if they are not applied sufficiently wet. An experienced job shop can do the job well, while a paint shop that has not used such a primer, will need to go through a learning curve.

Organic

zinc-rich primers will also perform well, and are easier to apply. However, they might not have the same corrosion resistance as the inorganic zincs.

Based on your actual requirements you should be able to pick from one of these options. Each one has its pro's and con's and you will need to make compromises regardless of the one you select.