Monitoring of Rouging and De-Rouging of Stainless Steel for Chemical Reactors by Photoelectrochemical and Impedance Measurements

The food and fine-chemical industry has for a long time been troubled by red brown to dark violet deposits in distillation columns, storage vessels and distribution systems for purified water and clean steam. Owing to the visual appearance of those deposits, the phenomena was referred as rouge. Although formation of rouge, i.e. rouging, is a commonly known phenomenon in stainless steel systems, the basic mechanism is still not fully understood. The phenomenon causes precipitation of iron as fine red-brown ferric oxide or hydroxide particles. Moreover adherent discoloration of the stainless steel surface is often observed in conjunction with the loosely adhered rouge particles. The affected systems are usually made of austenitic CrNiMo steel grade AISI 316L, which is the standard material for pharmaceutical and food plants. Although rouging has been known for decades the amount of research on this phenomenon is still very limited. One reason for this is the fact that rouging is a difficult and elaborate task to study experimentally in laboratory and require long experimental times. On the other hand, rouge removal is required to prevent both equipment and product purity, thus it is crucial to find a cost effective de-rouging process, and to evaluate the corrosion resistance of the SS surface soon after treatment. This work is focused on the physico-chemical characterization of passive films on AISI 316L SS mimicking rouged layers in order to get more information on the mechanism of rouging formation. We initially studied the electrochemical conditions (potential, bath composition, temperature and pH) inducing rouging on the surface of AISI 316L. The prepared passive films were characterized by Photocurrent Spectroscopy (PCS) and by impedance measurements in order to get information on their composition and on their electronic properties. Ellipsometric measurements were also performed in order to get information on the thickness of the films. We also tried to find an environmental friendly surface treatment able to remove rouging paying attention in characterizing the surface condition left after the cleaning step.