| Steel is the main structural material used in offshore engineering. Since seawater is recognized to be very corrosive for metals, it is necessary to study the corrosion and its protection of structural steel in seawater. Corrosion rate of steel in different kinds of seawater is an important corrosion parameter which is widely used in marine engineering. It is used not only as the basis of selecting materials in construction, but also as the foundation of construction design, and it is also can be used to predict the life-span of steel structures. Marine environmental factors play an important role in the corrosion process of metallic materials in seawater. Corrosion rate of carbon steel in seawater is determined by temperature,pH,dissolved oxygen,salinity and oxidation-reduction potential. In order to get reliable corrosion rate, the relationship between corrosion rate and the environmental factors of seawater state should be investigated. The routine methods of corrosion rate measurement such as weight loss, resistance method and electrochemical methods can't reflect the seawater state. Thus the more reliable method, soft-measurement method, was developed in recent work. Corrosion rate and its variety can be acquired by Soft-measurement method. It may be realized by measuring marine environmental factors such as temperature,pH,dissolved oxygen,salinity and oxidation-reduction potential, which can be gained fleetly and truly, then the relationship between corrosion rate and environmental factors is analyzed, thus mathematic model is built. Corrosion rate parameters ,whose truly in-situ measurement is difficult, can be calculated by the mathematic model. The advantages of soft-measurement lies in two sides, one is that it is able to provide marine environmental parameters according to corrosion rate and reflect the varieties in different conditions. Besides this, corrosion rate data can be got at once because the testing techniques of environmental parameters are fast and exact. In this thesis, the application of soft-measurement method in measuring corrosion rate of carbon steel in ocean environmental was studied. The main work ,results andconclusions are summarized as follows: (1)Corrosion rate depends on the mensuration and surface state of the samples. In order to get reliable corrosion rate, a lot of corrosion data were obtained by means of weight loss in lab,weight loss in natural sea,linear polarization resistance,potentiodynamic polarization and electrochemical impedance spectroscopy. And the relationship between testing results and mensuration or surface state was analyzed in contract to corrosion data from reference. These made it possible to analyze the effect of seawater state on corrosion rate and then to establish the soft-measurement method.The results show that: the factors contributing to corrosion rate include material, interface and seawater physical chemistry state. As the material and interface physical chemistry state are constant to certain measurement, the seawater state affecting corrosion rate is determined by temperature,pH,dissolved oxygen,salinity and oxidation-reduction potential,velocity,biologic activity,carbonate saturation,and so on. In this thesis, temperature,pH,dissolved oxygen,salinity and oxidation-reduction potential and velocity were discussed. (2)These environmental parameters are not independent. The interaction among them was investigated in this thesis, results show that:①temperature is an independent parameter, others except salinity are influenced by the changes of temperature；with the increasing temperature, dissolved oxygen content,pH and ORP decreases;② dissolved oxygen content in seawater is mainly affected by temperature and salinity, it decreases with increasing temperature and salinity;③ ORP in clean seawater is determined by dissolved oxygen content and pH, also affected by temperature. (3)The relationships between corrosion rate of carbon steels in seawater and several main corrosion environmen... |
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