Influence of the artificial patination on the atmospheric corrosion of Weathering Steel.

Abstract

Weathering steels (WS) have been widely used as a structural material for the last century. The main reason is their enhanced corrosion resistance unlike mild steel in atmospheres of moderate aggressiveness, due to the formation of a compact and adherent rust also called patina. WS have also been used in sculpture and as an ornamental material since the beginning of the 60s because of the attractive appearance of the patina. In order to obtain the appealing colors of the rust in a short time, sculptors and blacksmiths apply chemical compounds to the WS surface to accelerate the corrosion process. Nevertheless, these artificial patinas may compromise the natural evolution of the atmospheric corrosion process and prevent the formation of a well protective patina, which needs time to reach its protective ability. Therefore, the aim object of this work is to assess the influence of different artificial patination procedures on the subsequent atmospheric corrosion process. In authors' previous works, interviews with sculptors were carried out to know the treatments used in their studios. Several treatments were chosen and applied on WS coupons in the same way that sculptors work. These WS coupons were then exposed to an urban atmosphere in Madrid to analyze the evolution of the artificial patinas. This work presents the evolution after one year of atmospheric exposure of WS with previous treatments of high and low aggressiveness, based respectively on iron nitrate and acetic acid. Both of them were also compared with the rust formed on WS exposed to the same atmospheric environment without any previous treatment. Surface analyses of rusted WS were carried out by Raman spectroscopy and X-ray diffraction, as well as by color and thickness measurements. The corrosion behavior of the different patinas was assessed by Electrochemical Impedance Spectroscopy and mass loss. Cross section analyses were performed by micro Raman point analyses to identify the various phase structure of the different rust layers which form the patina. Two of the studied zones were chosen for a micro Raman mapping and also analyzed by Energy Dispersive Spectroscopy under Scanning Electron Microscope to determine the collocation of the alloying elements and the crystalline phases in the corrosion layers. The main difference between the two treatments is the rust composition. In the case of the iron nitrate treatment the main phase detected is goethite, whereas with the acetic acid treatment the main phase detected is lepidocrocite mixed with a less crystallized phase ferrihydrite. It has also been observed that Cr is usually associated with the presence of ferrihydrite. Compiling all this information it is possible to elucidate the effects that the artificial patination may have on the properties of the WS.