Corrosion Properties of High-Corrosion-Resistance Galvanized Steel Sheet, Posmac(R)

POSCO, one of the leading-edge steel makers in the world, recently launched a new high-corrosion-resistance galvanized steel product under the name of POSCO Magnesium Aluminum alloy Coating (POSMAC®). POSMAC® is considered to have superior corrosion resistance due to extra metal compositions in galvanic layer, which contains about 3% magnesium and 2.5% aluminum. POSMAC® with relatively thinner galvanic layer could replace heavy-galvanized steel thanks to excellent cut-edge corrosion properties. This new product is used for various applications and chromate or Cr-free coatings are applied to enhance corrosion resistance. Chromate coating is not favored nowadays for health and environmental issues. However, regulations regarding chromate usage are different for regions and trivalent chromate is still allowed even in some countries that prohibit hexavalent chromium. We tested several chromate coating compositions on POSMAC® and tried to determine best compositions for various applications. Darkening (blackening) as well as normal corrosion properties of chromated POSMAC® was tested according to chromate coating composition, especially hexavalent and trivalent chromium concentrations. Cr-free coating is mainly used for applications that require relatively better corrosion resistance against white rust formation such as electronic devices, where aesthetic property is also important. Corrosion resistance of Cr-free coating POSMAC® is enhanced by modifying coating compositions. Various techniques were used in this study to determine corrosion properties, i.e. salt spray test (SST), cyclic corrosion test (CCT), cut-edge corrosion test, etc. Several analytical techniques were used to investigate corrosion and anti-corrosion mechanism by chemical coatings.