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The protection of galvanised steel using a chromate-free organic inhibitor
Coatings; Corrosion protection; Steel Chromate conversion coatings; Chromate-free; Corrosion products; Corrosive medium; Degree of protection; Design/methodology/approach; Electrochemical test; Galvanised steel; Linear polarization; NaCl aqueous solution; Non-toxic alternatives; Open circuit corrosion; Organic corrosion inhibitors; Organic inhibitors; Phosphate conversion coatings; Protective properties; Salt fog test; Zinc coating; Zinc phosphates; Chromate coatings; Chromates; Corrosion inhibitors; Corrosion prevention; Corrosion protection; Corrosion rate; Corrosion resistance; Corrosion resistant alloys; Electrochemical corrosion; Electrochemical impedance spectroscopy; Galvanizing; Metal analysis; Phosphate coatings; Polarization; Sodium chloride; Stadiums; Steel; Testing; Toxic materials; Zinc; Zinc compounds; Protective coatings
Purpose - The purpose of this paper is to investigate corrosion protection provided by a water-soluble corrosion inhibitor as a non-toxic alternative to the chromate and phosphate conversion coatings on galvanised steel. Design/methodology/approach - Untreated galvanised steel samples were assessed simultaneously with galvanised steel samples treated with the chromate-free organic inhibitor, a conventional chromate conversion coating or a zinc phosphate modified with nickel, by means of immersion - weigh loss and electrochemical tests, using a naturally aerated 0.5 M NaCl aqueous solution as corrosive medium. In addition, selected superficial conditions of galvanised steel were submitted to a salt fog test. The electrochemical tests used were: open circuit corrosion potential logging, linear polarization, potentiodynamic polarization and electrochemical impedance spectroscopy. Corrosion products formed on samples withdrawn from the solution at different intervals were characterized using X-ray diffraction. Findings - All tests gave concordant results, indicating that the chromate-free inhibitor showed moderate protective properties in this electrolyte. Among the considered superficial conditions, the phosphate coating showed the most deficient performance. In all cases, it was observed that after moderately intense initial attack, the corrosion rate diminishes due to the formation and growth of insoluble corrosion product layers, which exhibit a passivating action. Research limitations/implications - The chromate-free organic corrosion inhibitor protected galvanised steel in this environment but the degree of protection was less than that provided by the chromate conversion coating. Originality/value - The paper presents an alternative to the toxic treatments with chromates, since the inhibitor works as an additional coating, sealing pores and other discontinuities found in the zinc coating. © 2009 Emerald Group Publishing Limited. All rights reserved.
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