Study On The Automatic Catalytic Corrosion Mechanism Of Steel Under Atmospheric Environment

The atmospheric corrosion of steel is such a serious problem that the study on it has always been concerned. But now, many scholars analyzed the phase and composition of the rust layer in the atmospheric corrosion process and explored the impact of various factors by exposuring to atmospheric corrosion experiments, atmospheric modeling experiments or electrochemical methods. From this point of view, the mechanism of steel corrosion was obtained. At present, the studies of corrosion process of steel by the way of the formation process ofγ-FeOOH,α-FeOOH and Fe₃O₄ from Fe²⁺ solution which is dissloved from Fe have been well investigated. However, it is never reported that there is the automatic catalytic mechanism in the steel corrosion process .In this paper, the process of steel corrosion in the rural environment was simulated by oxidating Fe(OH)₂ suspension with air at the neutral pH and low temperatures. Based on the above results, we put forward the auto-catalytic mechanism of Fe²⁺ in the process of oxidation and the concept that this mechanism can be applied in the rural atmosphere, industrial atmospheric and occeanic atmospheric corrosion process. The mechanism also was applied in how to explain the fuction of different metal ions of carbon steel. The results will be of great significance to the awareness and understanding of steel corrosion process.The main research aspects are summarized as follows.1.The air oxidation process of Fe(OH)₂ suspension at near-neutral conditions (pH = 6,7,8) and the effects of oxidation rate were studied. The results showed that the oxidation products of Fe(OH)₂ suspension with air isα-FeOOH andγ-FeOOH at near neutral pHs , and the oxidation rate law is slower with the simultaneously adding metheds, which is similar to the real environment of rural areas of steel corrosion.2.The capture process of ferrihydrite was studied. The results show that in order to capture ferrihydrite during the air oxidation process of Fe(OH)₂ suspension, phosphate should be added. The success capture of ferrihydrite will provide a favorable support to the autocatalytic mechanism of Fe²⁺, and it can further extend this mechanism to the atmospheric corrosion of steel. 3.Three different atmospheric corrosion of steel simulation were studied. It was found that corrosion products were different in different circumstances, but the corrosion products all containedγ-FeOOH andα-FeOOH. This shows thatγ-FeOOH andα-FeOOH is the main corrosion products, which are consistent with the oxidating products of air oxidation of Fe(OH)₂ suspension at near neutral conditions. Combined with corrosion mechanism at the atmospheric environment proposed in this paper, it is thought the Fe²⁺ acts as an automatically catalyst in the process of atmospheric corrosion of carbon steel. Owing to Fe²⁺ comes from the steel itself, which means steel exists autocatalytic corrosion mechanism under atmospheric environment.4.In order to simulate the corrosion process of alloy steel, the air oxidation process of Fe(OH)₂ suspension in the presence of Cu²⁺, Ni²⁺, Cr³⁺, Mn²⁺ ions at pH 7 and 8 were studied. The results showed that all the added metal ions impedes the formation ofα-FeOOH andγ-FeOOH to a certain extent. In sum, from the point of oxidation process, the presence of such metal ions as Cu²⁺, Ni²⁺, Cr³⁺, Mn²⁺ not only extended the reaction times but also retarded the oxidation process as well as the automatic catalysis of Fe²⁺.