Zn and Zn±Sn alloy coatings with and without chromate layers. Part I: Corrosion resistance and structural analysis L. SZIRA Â KI 1 *, A Â . CZIRA Â KI 2 , Z. VE Â RTESY 3 , L. KISS 1 , V. IVANOVA 4 , G. RAICHEVSKI 4 , S. VITKOVA 4 , S. and Ts. MARINOVA 5 1 Department of Physical Chemistry, Eo Ètvo Ès L. University, H-1518 Budapest 112, PO Box 32, Hungary 2 Department of Solid State Physics, Eo Ètvo Ès L. University, H-1088 Budapest, Mu Âzeum krt.6-8., Hungary 3 Research Institute for Materials Sciences, Hungarian Academy of Science, H-1525 Budapest, PO Box 49, Hungary 4 Institute of Physical Chemistry, Bulgarian Academy of Science, So®a 1040, Bulgaria 5 Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, So®a 1040, Bulgaria (*author for correspondence, e-mail: sziraki@ludens.elte.hu) Received 19 May 1998; accepted in revised form 8 December 1998 Key words: conversion coatings, EIS, TEM, XPS, Zn corrosion, Zn±Sn alloy corrosion 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 Na 2 SO 4 =pH 5 solution using a.c. impedance and d.c. polarization techniques. The polarization curves revealed that the chromate layers in¯uence 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 ®lm 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 pro®le of the corroded surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS). 1. Introduction Chemical chromating is often used to enhance the corrosion stability and/or paint adhesion of Zn plates or galvanized steel. The active/passive type of corrosion behaviour observed on Zn±Sn alloy with 0  80 wt % tin content [1] is also expected to be improved by chromating. The thermal Zn±Sn alloy with tin content up to 80% is sacri®cial to steel in slightly acidic and neutral salt solution [1], because zinc maintains the corrosion potential at a more negative value than does pure tin but the Zn loss from the alloy is moderated by the the accumulated porous zinc and tin oxides. The corrosion resistance of the Zn±Sn alloy strongly depends on its composition and the corrosion medium. Although the corrosion resistance of the Zn±Sn alloy is not actually lower than that of pure Zn, Sn or Cd, it is often used as an alternative for these metals from various practical and economical considerations [2±5]. Earlier we reported a series of studies on these problems. Zn±Sn alloy coatings containing 10±30% tin were deposited from a slightly acid electrolyte and the deposition conditions were optimized [6]. We proposed three compositions with appropriate Cr(VI) activating ion ratios for chromating of electrodeposited Zn±Sn alloys with low Sn content. The results [7] of the accelerated electrochemical Paatsch test and a salt spray chamber test indicated an increasing protective ability of the chromate ®lms in the following sequence: colourless (CL) on Zn±Sn > iridiscent yellow (IY) on Journal of Applied Electrochemistry 29: 927±937, 1999. 927 Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands.