Article

Journal of Applied Electrochemistry

, Volume 29, Issue 8, pp 927-937

Zn and Zn–Sn alloy coatings with and without chromate layers. Part I: Corrosion resistance and structural analysis

  • L. SZIRÁKIAffiliated withDepartment of Physical Chemistry, Eötvös L. University
  • , Á. CZIRÁKIAffiliated withDepartment of Solid State Physics, Eötvös L. University
  • , Z. VÉRTESYAffiliated withResearch Institute for Materials Sciences, Hungarian Academy of Science
  • , L. KISSAffiliated withDepartment of Physical Chemistry, Eötvös L. University
  • , V. IVANOVAAffiliated withDepartment of Solid State Physics, Eötvös L. University
  • , G. RAICHEVSKIAffiliated withInstitute of Physical Chemistry, Bulgarian Academy of Science
  • , S. VITKOVAAffiliated withInstitute of Physical Chemistry, Bulgarian Academy of Science
  • , S. MARINOVAAffiliated withInstitute of General and Inorganic Chemistry, Bulgarian Academy of Science
  • , T. MARINOVAAffiliated withInstitute of General and Inorganic Chemistry, Bulgarian Academy of Science

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Abstract

The aqueous corrosion resistances of Zn and Zn–Sn (∼ 20 wt% Sn) electrodeposits, passivated by immersion in chromating solution with different ratios of Cr(vi) to activating ions, are compared. The electrochemical behaviour of various chromated and nonchromated coatings were investigated in deaerated 0.5 mol dm−3 Na2SO4/pH 5 solution using a.c. impedance and d.c. polarization techniques. The polarization curves revealed that the chromate layers influence both the cathodic and anodic reactions. The corrosion rate of each specimen decreases with time due to the accumulation of corrosion products. The dark yellow (DY) chromate film on the Zn–Sn alloy and the iridiscent yellow (IY) on Zn yields the best protective ability in agreement with the assessment of prolonged salt spray chamber tests. These chromate layers resembling cracked mud become permeable to the electrolyte after immersion and, as a consequence of the transformation and the leaching of certain Cr compounds, a very porous agglomerate of corrosion products forms. The morphology and structure of dark yellow chromated Zn–Sn alloy was also investigated by transmission electromicroscopy (TEM) and scanning electronmicroscopy with microprobe (SEM/EDS) analyses before and after corrosion. The depth profile of the corroded surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS).

conversion coatings EIS TEM XPS Zn corrosion Zn–Sn alloy corrosion