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Saarimaa V.,Top Analytica Oy | Kauppinen E.,Ruukki Metals Oy | Markkula A.,Ruukki Metals Oy | Juhanoja J.,Top Analytica Oy | And 2 more authors.
Surface and Coatings Technology | Year: 2012

Cr-containing conversion layers for hot dip galvanized steel are currently being replaced with more environmentally friendly options. Generally, the hexavalent chromium pretreatments are known to provide excellent corrosion protection in most environments, while coil coated products pretreated with Cr-free alternatives are more prone to corrosion. Surface sensitive analysis techniques were employed on pretreated HDG in order to clarify the importance of mechanical distribution of a Ti-based pretreatment layer to corrosion protection. A correlation between the conversion layer evenness and the humidity resistance of the corresponding painted samples was observed. The studied Ti-based pretreatment layer was thus confirmed to provide protection mainly via barrier mechanism. Defects in the microscale distribution of the pretreatment layer are not detected by off-line X-ray fluorescence measurements that are commonly used for determination of coating weights of pretreatments on HDG. Automated EPMA measurements can provide valuable information on the distribution of trace elements in Cr-free conversion layers, which can be used as an indication of the conversion layer evenness and further utilized in process optimization. A transition from Cr-containing pretreatments to Cr-free pretreatments requires careful cleaning of the substrate and well managed application of the pretreatment film. However, when proper distribution and reaction of the pretreatment solution are assured, humidity resistance equivalent to that of Cr-containing pretreatments can be expected. © 2012 Elsevier B.V.

Saarimaa V.,Top Analytica Oy | Markkula A.,SSAB | Juhanoja J.,Top Analytica Oy | Skrifvars B.-J.,Top Analytica Oy
Surface and Coatings Technology | Year: 2015

Hot dip galvanized zinc coatings (containing about 0.2% of aluminum) were studied with surface analytical techniques. A few nanometer thick aluminum oxide (Al2O3) layer dominated the very outermost surface of such zinc coatings. The inert Al2O3 layer formed a continuous impermeable barrier on zinc, providing short term humidity resistance. However, this barrier effect was largely lost during temper rolling. Residues of Al2O3 were shown to prevent titanium hexafluoride pretreatment chemical from reacting with the zinc surface. Segregated aluminum/iron (Al/Fe) intermetallic compounds were also detected at the surface of the zinc coating. The Al/Fe compounds populated the surface immediately beneath the Al2O3 layer. The Al/Fe precipitates were found predominantly in the dendritic valleys. Repulsion between the Al/Fe precipitates and the pretreatment chemical was observed. © 2015 Elsevier B.V.

Saarimaa V.,Top Analytica Oy | Markkula A.,SSAB | Juhanoja J.,Top Analytica Oy | Skrifvars B.-J.,Top Analytica Oy
Journal of Coatings Technology Research | Year: 2015

Humidity resistance of pretreated and coil-coated panels was evaluated with a special emphasis on the process parameters that govern the uniformity and barrier properties of hexafluorotitanic acid-based pretreatment layers. Humidity resistance tests showed that good barrier effect was obtained with pretreatments in the coating weight range of about 2–17 mg Ti/m2. It was further verified by rinsing experiments that a fully reacted pretreatment layer is achieved at the very low end of the optimal coating weight range. Increasing the coating weight leads to the formation of precipitated, loosely bonded porous structure, which was detrimental to humidity resistance. In addition to the coating weight, the microscale pretreatment layer uniformity is of utmost importance for good barrier properties. Electron probe microanalysis measurements provide an efficient tool to assess the pretreatment layer uniformity and facilitate monitoring of process adjustments with clear correlation to humidity resistance properties of painted panels. The surface condition of the hot dip-galvanized steel and the material of the application rolls were found to strongly affect the uniformity of the pretreatment layers. © 2015, American Coatings Association.

Saarimaa V.,Top Analytica Oy | Manni J.,Top Analytica Oy | Kauppinen E.,Ruukki Metals Oy | Markkula A.,Ruukki Metals Oy | And 2 more authors.
Surface and Interface Analysis | Year: 2014

Hexavalent chromium containing pretreatments and primers for coil coating are soon to be entirely prohibited, which sets new demands for Cr-free alternatives. Most of the presently used Cr-free pretreatment layers operate predominantly via barrier formation and adhesion promotion mechanisms and lack the self-healing effect typical for Cr6+-pretreatments. This sets new demands also for the formation and monitoring of these layers. The barrier thickness and chemical composition of Cr-free pretreatment layers on hot dip galvanized steel were studied using cross sections from broad ion beam (BIB) sample preparation and ultramicrotome cutting. BIB milling provided finely polished cross sections of pretreated samples. Film thicknesses of 20-50 nm were accurately determined for Cr-free pretreatments containing 4-10 mg Ti/m 2 using BIB milling and scanning electron microscopy imaging. Scanning transmission electron microscopy, integrated with aberration correctors and X-ray energy dispersive spectrometry, of an ultramicrotome cut pretreated and painted samples provided detailed chemical information. Metal complexes were detected close to the pretreatment/zinc interface, while the polymeric part of the pretreatment layer prevailed closer to paint. Copyright © 2014 John Wiley & Sons, Ltd.

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