Coating weight refers to the amount of zinc coating applied to a product for a given surface area. Two different methods can be used to measure the coating weight of hot-dip galvanized steel.
The first method to measure the coating weight is a process called weigh-galvanize-weigh, and is only appropriate for single specimen samples. This method measures the weight of a steel part after it has been cleaned, and again after galvanizing is completed. Weigh-galvanize-weigh will only measure the added zinc weight in the coating, but the actual coating is made up of both iron and zinc, so this method will underestimate the total coating weight by 10%. Furthermore, it can be very difficult to measure and calculate the surface area of a complex steel fabrication, and this makes coating weight values even less accurate.
Weigh-strip-weigh is the second method used to measure coating weight, and again is destructive so it is only appropriate for single specimen samples. This process measures the weight immediately after a galvanized part is cooled, then stripping all of the zinc coating and weighing it again. The difference in the weights is then divided by the surface area of the part to find the amount of zinc coating added during hot-dip galvanizing. This method renders a part unusable as it removes all of the coating during the measurement. Weigh-strip-weigh is usually only used on small products such as nails, and can be inaccurate because when the coating is stripped, there may be some base metal stripped along with the zinc coating. The extra iron that is potentially included in the measurement will lead to an overestimation of the actual zinc coating weight.