Quantitative phase analysis of galvannealed coatings by coulometric stripping

The characterization of different Fe-Zn phases present in a galvannealed coating has been a challenging area of research in recent years. There have been doubts about the validity of the results obtained by electrochemical stripping because the data cannot be used for straightforward phase identification and because quantification needs to take into account the effect of various electrochemical side-reactions. The optimization of the electrochemical stripping method (also known as coulometric dissolution) and its use as a routine analysis method is reported. It is shown that the method allows the determination of the total Fe concentration, and the thickness and composition of the eta, zeta, delta and Gamma phases. Coulometric data obtained on a large number of TiNb interstitial free (IF) galvannealed samples were compared with the total iron and zinc concentrations determined by ICP-MS. Validation of the results was done by ICP-MS and EPMA. The coulometric method was verified on a large number of industrially produced galvannealed coatings and pilot line simulated galvannealed coatings.