To solve the problems of high temperature and non-uniformity of coloring on stainless steel, a new chemical coloring process, applying ultrasonic irradiation to the traditional chemical coloring process, was developed in this paper. The effects of ultrasonic frequency and power density (sound intensity) on chemical coloring on stainless steel were studied. The uniformity of morphology and colors was observed with the help of polarizing microscope and scanning electron microscopy (SEM), and the surface compositions were characterized by X-ray photoelectric spectroscopy (XPS), meanwhile, the wear resistance and the corrosion resistance were investigated, and the effect mechanism of ultrasonic irradiation on chemical coloring was discussed. These results show that in the process of chemical coloring on stainless steel by ultrasonic irradiation, the film composition is the same as the traditional chemical coloring, and this method can significantly enhance the uniformity, the wear and corrosion resistances of the color film and accelerate the coloring rate which makes the coloring temperature reduced to 40°C. The effects of ultrasonic irradiation on the chemical coloring can be attributed to the coloring rate accelerated and the coloring temperature reduced by thermal-effect, the uniformity of coloring film improved by dispersion-effect, and the wear and corrosion resistances of coloring film enhanced by cavitation-effect. Ultrasonic irradiation not only has an extensive application prospect for chemical coloring on stainless steel but also provides an valuable reference for other chemical coloring.

A light reflection device combined with visible light spectrophotometer was used to measure the reflectance-wavelength curves of colored stainless steel after coloring, hardening and sealing. After these processes, the coordinate in CIE 1931 color coordinate system and brightness of samples were confirmed by chromatometry method, and the effects of hardening and sealing processes on color were determined. In hardening process, the thickness of coloring film increases significantly, the peaks of reflectance-wavelength curves move to the long-wave region, the hue of samples changes clockwise in CIE 1931 coordinate diagram, and the trend of saturation in hardening process is consistent with the saturation of the sample having larger potential difference. In sealing process, the thickness of film increases slightly, the peaks of reflectance-wavelength curves move to the longwave region, the reflectivity and brightness of samples increase, the hue of samples moves clockwise in CIE 1931 coordinate diagram, and the saturation of some samples decreases. Stainless steel has the advantages of high temperature resistance, corrosion resistance, long service life, 100% recyclable and harmless to human health. By 2019, the output of stainless steel in the world has reached 52.21 million tons, becoming one of the most important alloy materials. As a kind of decoration material, it needs bright-coloured and diverse appearance. The stainless steel coloring process can well meet this requirement. The coloring methods of stainless steel include chemical coloring 1-5 , electrochemical coloring 6,7 , physical vapor deposition 8,9 , high temperature oxidation, laser pulse 10-12 , etc. Among them, the chemical coloring method, also known as International Nickel Corporation (INCO) coloring, is the most widely used method because of its bright colors 13 , excellent wear resistance 14 , which have been widely used. As decorative materials, color analysis on colored stainless steel samples is very important to their application. Musa et al. 13 have measured the reflectivity of chemically colored stainless steel samples, their optical properties have been obtained, the results show that heating and covering by Si 3 N 4 film could improve the absorption of sunlight. Vazquez-Santoyo et al. 15 have measured wavelength-reflectance curves of colored stainless steel samples, and the colors of stainless steel samples have been expressed in Commission Internationale de l´Eclairage (International Commission on illumination, CIE) coordinate system by using chromatometry calculation. It can be seen that colors of stainless steel samples can be effectively expressed by reflectivity curves and CIE coordinate system. In our previous research 5 , we have applied the chromatometry principles on color analysis of different colored stainless steel samples, explored the brightness and the color cyclic change regularity of colored stainless steel with different color potential differences, and the color of samples are analyzed by CIE coordinate and munsell coo...