| Cr coatings are widely used for decoration due to their attractive brightness, highcorrosion resistance and excellent wear resistance. Conventional Cr coatings areelectrodeposited from hexavalent Cr electrolyte. However, hexavalent Cr electroplatinghas been restricted from industry due to the intense toxicity and carcinogenicity ofhexavalent Cr. Trivalent Cr electroplating has been considered as a promising alternativeto hazardous hexavalent Cr electroplating. Many trivalent Cr electroplating arts havebeen developed since commercial trivalent Cr plating was introduced in the US in1976.However, thick Cr coating with acceptable quality is hardly obtained from trivalent Crelectrolytes; thus, it is only commercially used for decorative Cr electroplating. thecorrosion resistance of trivalent chromium plating can not comparable with hexavalentchromium plating, so, Researching on how to improve the corrosion resistance oftrivalent chromium coating play a very important role in the development process oftrivalent chromium electroplating.In this paper, we research the corrosion resistance of trivalent chromium coatingand hexavalent chromium coating which is provided by some company electroplated onplastics. In order to study the difference of corrosion resistance between trivalentchromium coating and hexavalent chromium coating. We did artificial rain waterimmersion test and neutral indoor sulfur dioxide accelerated corrosion test and neutralsalt spray corrosion test. Other more, The electrode potentials were measured against asaturated calomel electrode (SCE) in artificial rain water. the surface morphology ofelectrodeposited chromium layers was scanned by electron microscopy (SEM), Atomrelatively content and valence was investigated by X-ray photoelectron spectroscopy(XPS), The structure of the chromium coating was investigated by transmission electronmicroscopy (TEM).The study shows that:(1) the presence of micro-cracks in the trivalent chromiumplating coatings can improve corrosion resistance, but the corrosion resistance of thetrivalent chromium coating can’t comparable with hexavalent chromium coating, itmainly because of the trivalent chromium contains impurities of iron, at the same time, ithas been shown that C, O, N can occur within the structure of amorphous Crelectrodeposits which increase the open potential of the chromium plating, in copper/nickel/chromium composite coating system, High open-circuit potential of chrome-plated will accelerate the corrosion of the underlying plated nickel layer, then,corrosion product of nickel-plated will increase chrome plating’s off, at last, chromiumcoating loss the protection for the entire coating system.(2) the microstructure of theplating will influence the corrosion resistance of trivalent chromium.,micro-cracks caneffectively disperse corrosion current. In the same time, iron content as a impurity willreduce the corrosion resistance of trivalent chromium plating. the iron electrode potentiallower than the nickel’s, so, it priority to corrosion as the anode, and it will destruct of thecoating structure, resulting in poor corrosion resistance of trivalent chromium plating atthe both ends part. |
