Repairing the cracks network of hard chromium electroplated layers using plasma nitriding technique

• EDS charts and tables have been added to manuscript.
• XRD tests and reports have been revised again.
• SEM micrographs of CPN samples have been changed.

The use of hard chromium electroplated coatings for industrial and decorative applications is widespread. However, there seem to be some defects like the existence of an extensive micro-cracks network that restrict their applications. They affect corrosion resistance due to formation of direct routes to the substrate through which corrosive agents could reach the main metal. Plasma nitriding process is proved an effective technique to overcome such problems. The objective of this study is to investigate the effect of plasma nitriding parameters namely, time and temperature, on the closure of the cracks. Samples of a hard chromium electroplated hot work tool steel were heat-treated via conventional plasma nitriding (CPN) and active screen plasma nitriding processes (ASPN) at 500 and 550 °C for 5 and 10 h, and at 550 and 600 °C for 10 and 15 h, respectively. The morphology and microstructure of the samples were characterized by the scanning electron microscopy (SEM) imaging. The coating phases were determined by the X-ray diffraction (XRD) method. The corrosion properties of samples were evaluated using anodic polarization tests in a 3.5 wt% NaCl solution at 25 °C. The results showed that both techniques filled up the cracks. However, the closure mechanisms were different. In CPN method, the cracks were disappeared due to the transformation of chromium to its nitrides, which occur with volume expansion. It is worth noting that this phenomenon occurs at lower temperatures and shorter times compared to ASPN method. In ASPN method, the iron nitrides detached from active screen were deposited inside the cracks to fill them up. Moreover, it was found that the corrosion resistances of the samples modified by the CPN process were superior to the ASPN samples.