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Mono‐carboxylate conversion coatings for AZ31 Mg alloy protection
Author(s) -
Frignani A.,
Grassi V.,
Zucchi F.,
Zanotto F.
Publication year - 2011
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.200905615
Subject(s) - carboxylate , dielectric spectroscopy , aqueous solution , stearic acid , coating , materials science , lauric acid , solubility , electrolyte , magnesium , electrochemistry , alkyl , inorganic chemistry , nuclear chemistry , chemistry , organic chemistry , electrode , metallurgy , nanotechnology , fatty acid , composite material
Conversion coatings on a magnesium alloy were obtained by dipping AZ31 specimens in aqueous solutions of sodium salts of mono‐carboxylic acids (stearic, palmitic, myristic, lauric, mono‐carboxylate ion concentration from 1 to 5 mM, depending on the salt solubility) for 24 and 72 h at room temperature, or 24 h at 50 °C. The influence exerted by the treatment time, bath temperature and alkyl chain length on the efficiency of these coatings was studied. The performances of the coatings were evaluated by potentiodynamic polarization curve recording after 1 h immersion in 0.05 M Na 2 SO 4 solution, while their temporal evolution was monitored by electrochemical impedance spectroscopy (EIS) spectra during 24 h. Further and long lasting tests were carried out also in 0.1 M NaCl solution. The efficiency of the coatings depended on the aliphatic chain length, and increased as the treatment time and the bath temperature were increased. The coating of lower homologue only hindered the cathodic process, while those of the higher homologues markedly inhibited the anodic process too. The best performances were displayed by 24 h‐50°C stearic conversion coating, which maintained a very high efficiency for over 800 h immersion in 0.05 M sulphate solution.