Inspecting Coating Thickness & Continuity

As a final step in the galvanizing process, the hot-dip galvanized coating is inspected for compliance with specifications. The coating thickness is the single most important component in determining a galvanized coating’s quality. Coating thickness, however, is only one inspection area. Coating uniformity, adherence, and appearance should also be checked. Inspection of the galvanized product can be most effectively and efficiently conducted at the galvanizer’s plant where questions can be asked and answered quickly.

There are a number of simple magnetic gauges that can be used to give a convenient and reliable measurement of the zinc coating thickness, provided the instruments are properly calibrated. The three most common types of metal coating thickness gauges are:

  • Magnetic balance gauges, sometimes called “banana gauges,” measure variation in the force of attraction between two ferromagnetic bodies as a function of the distance between them. This type of gauge has the advantage of being able to measure the coating thickness in a horizontal or vertical position.
  • Pull-off magnetic gauges are also based on magnetic attraction to the underlying steel. These devices are shaped somewhat like a pen and are very convenient to make quick, rough estimates to determine whether the coating thickness meets specification.
  • Electronic gauges are the easiest and most accurate coating thickness measurement gauges available. They have the ability to connect to an assortment of probes, providing the ability to measure on any orientation.

One of the major advantages to specifying hot-dip galvanized steel is the ease of identifying coating defects immediately after galvanizing. Any areas that may remain uncoated are easily identifiable. If large areas (see ASTM A 123) of the steel remain uncoated due to residues left on the steel from fabrication, then the steel must be stripped free of zinc and processed again. If small areas are left ungalvanized, they can be reconditioned using one of the three accepted methods of touch-up and repair, (see ASTM A 780).

Different coating appearance may result when different steels are galvanized. Listed below are links to other portions this program will explain differences in acceptable coating appearances that, despite their different appearance, will provide long-term corrosion protection.