Galvanized Steel, Painted Galvanized Steel and Failure Analysis
Hot dip galvanizing has been an attractive and economical means of corrosion protection for construction, utility tubular and lattice structures. Galvanized steel components are protected from corrosion attack due to both barrier effect and also due to galvanic (sacrificial) action of zinc. Zinc does a fine job of protecting a steel components in moderately corrosive and extremely corrosive environments. It provides long term protection both above ground and underground portion of structures .
Typically and for best performance the steel should conform to the mechanical and chemical properties listed in American Society for Testing and Materials (ASTM) specification A572. We also recommend the maximum silicon content for steel substrate be 0.06 % to ensure an adequate free zinc and uniform galvanized finish. The mechanical strength requirements for structural performance, such as tensile strength, (assuming the inherent material strength remains constant), is then dependent upon the material cross sectional area. If inadequate, tensile failures could occur at locations where corrosion has produced localized reductions in cross sectional areas and created stress raisers. Higher tensile strength steels have less ductility and toughness. These steels are considered notch sensitive. Normal constructional steels would not typically be notch sensitive but high strength low alloy (HSLA) steels can be notch sensitive. Corrosion pitting can create the notch which then becomes the location of crack initiation. Pitting or reduced thickness areas due to corrosion can also initiate mechanical fatigue cracks.
We have inspected galvanized roofs, galvanized pipes, galvanized lattices in service which date back to early 20th century. Upon inspection we found out that galvanized layer is present even after 100 years of service. The key point in long service life is that the soil in that location provided the protective layer on galvanized surface. However, both white rust and paint failures have been observed on galvanized steel components in service after few years in service.
Not all galvanizing facilities are the same, however, and some times the quality of the galvanized layer is compromised due to lack of QC in the process. The production of high quality galvanized steel strctures depends on the metallurgical reaction between steel and molten zinc. The micro-structural characteristics, grain structure (spangle formation), surface segregation and corrosion of galvanized coatings depend on both steel and bath compositions, coating processes and post-coating processing. Factors often associated with corrosion failure of galvanized steel components are improper thickness, excessive brittle inter-metallic alloy layer, galvanizing, substrate surface preparation if coated, storage conditions, water/air/soil service conditions, or unsuitable coating selection for corrosive applications.
Upon your request we can inspect and assess your galvanized steel components both in field and laboratory and determine the primary cause for accelerated corrosion or cracking.