Cathodic Protection on Bare Spots of Galvanized Steel
My customer damaged the galvanized coating on some steel during transport to the job site. What is the largest bare area on galvanized steel that will be cathodically protected?
One of the reasons galvanizing is chosen as a protective coating is because of its outstanding abrasion resistance, but the coating can be gouged or damaged if handled roughly. When possible, damaged galvanized coatings should be touched up, per ASTM A780. However, there are times when a damaged galvanized coating cannot be touched up, so it is good to know how much cathodic protection the galvanized coating offers. The design of galvanized steel is such that the zinc coating (anodic material) covers the steel (cathodic material). This pairing of an anodic material to a cathodic material is intentional and allows the zinc coating to corrode sacrificially in place of the steel corroding when in the presence of an electrolyte. This sacrificial action is known as cathodic protection and the anode, cathode, electrolyte and returns current path are known as a bimetallic couple.
In terms of cathodic protection offered by a galvanized coating of adequate thickness to a bare area, the two most important variables are the width of the bare area and the electrolyte that connects the galvanized coating and bare area. The properties of the electrolyte determine the maximum width of a bare area that will be cathodically protected. Electrolytes vary in resistivity, thickness, and how quickly they dry electrolytes with lower resistivities allow a higher amount of current to flow through the bimetallic couple and therefore tend to increase the bare area that will be cathodically protected. If an electrolyte is very thin or dries quickly, the amount of current that runs through it can be minimal, which then limits the size of the area cathodically protected on bare areas?
Due to the variability in electrolyte properties, it is hard to give exact widths of bare areas on galvanized steel that will be cathodically protected, but various studies have found in atmospheric exposure in urban environments, bare areas up to 2 millimeters in width will be completely cathodically protected. Larger bare areas can receive partial cathodic protection. Another study found the width of full cathodic protection to be up to 5 millimeters in industrial-marine climates. In immersion applications, the amount of cathodic protection offered by a galvanized coating tends to be greater because the current flow between the two metals tends to be larger. In fact, zinc anodes are commonly attached directly to ship hulls for the purpose of cathodic protection.
In summary, galvanized coatings do offer some cathodic protection to damaged areas; however, the amount of cathodic protection depends on the properties of the electrolyte present in the bare area.