Monitoring of a Zr-based conversion coating on galvanised steel and its performance against corrosion

Abstract

New protective coatings with satisfactory performance and low impact are required by the surface treatment industry as effective Cr(VI)-free substitutes. Recently conversion coating systems containing Zr/Ti have shown great promise particularly for zinc finishes on steel and in this paper, we study the influence of bath conditions (e.g. pH and time) on conversion layers composed of a Zr-based film. The coating formation was studied by simultaneous monitoring of open circuit potential and surface pH using a novel cell design. Samples were subsequently characterised by electrochemical impedance spectroscopy in NaCl over a 24-hour period. The system demonstrates a correlation between pH of formation and corrosion resistance.

Influence of Concentration and pH of Hexafluorozirconic Acid on Corrosion Resistance of Anodized AA7075-T6

“…But, if this pH is too low, as 2.5, the anodic oxide will be excessively deteriorated, losing part of its capability of protection instead of anchoring the Zr conversion coating, as it was desired. On the other hand, if the pH is higher than necessary, the solution is not acidic enough to slightly dissolve the anodic oxide to initiate the reaction on the surface and, instead, the ZrO 2 will form and precipitate in the solution bulk 27 . This last situation happened to the H 2 ZrF 6 solution with pH 4.…”

“…The concentration of H 2 ZrF 6 solution is also important to the usability of the Zr conversion coating over anodized AA7075 alloy. Solutions with low concentrations are commonly utilized directly on other substrates 17,19,27 but have now shown to be inappropriate for being used as a corrosion protective on anodized layers. There are studies evaluating corrosion resistance of different conversion coatings based on Zr applied directly on AA7075 alloy 4,19,28 .…”

“…Nevertheless, the solutions are complex: containing a mixture of many inorganic salts 4 or even commercial baths 28 , making it difficult to compare the concentration of each component and its relevance to the deposition mechanism. On the other hand, studies using only H 2 ZrF 6 were carried out for other substrates, as galvanized steel 27 and other aluminum alloys 17,19 . Thus, considering the importance of the material composition on the deposition process of conversion coatings, it is possible that AA7075 alloy needs a higher concentration of Zr so that the deposition is possible.…”

Aluminum Protection by Using Green Zirconium Oxide Layer and Organic Coating: An Efficient and Adherent Dual System

“…The samples treated with H 2 ZrF 6 showed the highest corrosion resistance compared to the unsealed aluminum samples. They also applied the same strategy to galvanized steel substrates, proving that the Zr-CCC deposition is highly dependent on the solution pH and on the reaction time [20]. Such pre-treatments have demonstrated to improve both the corrosion resistance and the adhesion of top coats and adhesives [9,10,[21][22][23][24].…”

Green Nanocoatings Based on the Deposition of Zirconium Oxide: The Role of the Substrate

“…Such coatings, with specific weight of only 0.01 to 0.02 g/m 2 , are extremely thin and represent a great advantage of cost and light-weighting final material for the producers [ 1 ]. The effectiveness of the dip-coating process by immersion of the metal substrate in hexafluorozirconic-acid-based (H 2 ZrF 6 -based) solution is pH limited and the homogeneity of the obtained films is sometimes poor [ 12 ]. Previous works have demonstrated that the incorporation of auxiliary additives in the bath solutions, like copper (II), nickel (II), nitrate (NO 3 − ) and hydrogen phosphate (HPO 4 2− ) ions, are necessary to further improve the barrier property and the density of Zr-CCC layer [ 13 , 14 ].…”