Chromate-free zinc conversion coatings characterised by grazing incidence X-ray diffractometry

Alternative processes for chromate conversion coatings on zinc represent a research challenge for environmental reasons. A potential concept yielding a promising corrosion behaviour without the need for toxic chemical additiv es is the application of alternating currents (AC) together with the additi on of inorganic nitrogen compounds in an electrochemical bath. A first graz ing incidence X-ray diffractometry (GIXD) study of the composition. and cry stallinity of this novel conversion coating system is presented. GIXD has b een employed because this technique has proved to be successful in the anal ysis of electrochemically formed conversion coatings down to 10 nm thicknes s. Various ion additives like NO2-, NO3- and NH4- in NaOH solution were app lied in the AC treatment. The nitrite anion. NO2, yielded the best result. It increased the generation rate and crystallinity of the passive layers. T he ratio of the crystalline ZnO to zn(OH)(2) content could be influenced by the AC amplitude and the AC cycling time. GIXD decisively contributed to t he improved understanding of this chromate-free zinc conversion process.