Stainless steel, corrosion measurement polarization resistance

Linear polarization resistance (LPR). These measurements allow the actual corrosion rate of embedded probes or of the reinforcing bars to be monitored over time. The measurement principle is described in Section 16.2.3. In addition to the reference electrode a counter-electrode of a corrosion resistant material (e. g. stainless steel or activated titanium) has to be embedded. Several compact LPR sensor systems were developed and installed in structures such as precast deck elements in a road tunnel [6,20]. When existing structures have to be monitored for corrosion rate, a corroding piece of rebar can be isolated (by cutting) to get... 

ASTM G59 descrihes a method for conducting potentiodynamic polarization resistance measurements [14]. It descrihes a setup for type 430 stainless steel in sulfuric acid, as for ASTM G5 described earher, and the same apparatus is specified. According to this standard, the potential should he scanned from 30 mV negative of the corrosion potential to 30 mV positive of the corrosion potential at a rate of 0.167 mV s . ... 

Linear polarization resistance can be applied to corrosion systems with electrochemical activation control, such as carbon steels and some stainless steels in low concentrations of sulfuric acid. For corrosion systems with a mass transport control or passivating systems, such as carbon steels in water with a pH between 5 and 10, the hnear polarization equation is not valid. Additionally, the normal fluctuation of corrosion potential during the measurements can significantly affect the accuracy of the measurements. Under most circumstances, a larger polarization than 10 mV may be used to increase the signal/noise ratio. However, it may put the system out of the linear region, introducing some additional errors in the measurements. 

Cyclic anodic polarization procedures based upon ASTM G5 (Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) have been used to evaluate the localized corrosion resistance of stainless steel alloys in paper machine white waters. The difference between the open circuit or naturally occurring corrosion potential and the pitting breakdown potential of various materials has been reported by many investigators [lO-Id]. Bowers [14] called the difference between the breakdown potential and the potential of the cathodic/anodic current reversal the margin of safety. He also noted the effect of the sulfate to chloride concentration ratio on localized corrosion of Types 304 and 316L stainless steels, These results permitted alternative materials of construction to be ranked and their limits of resistance to be defined. 

Audouard et al. and Dupoiron [758-760] simulated the p)er-formance of stainless steels in batch digesters during filling after a cook. Their studies were conducted under heat transfer conditions where white liquor was sprayed onto metal surfaces heated above the boiling point. They measured corrosion potentials, redox potentials, and polarization resistance and performed polarization tests in white and black liquors and mixtures of both at temperatures up to 170°C. 

The treatment conditions are the same as those in Fig. 7. The electrochemical test results for Hybrid-NCT, Nitriding-NT, Carburizing-CT were described in Fig. 12 The NT and CT showing that the current density of treated stainless steel were decreased in the anodic region which indicating positive effect regarding the improvement of corrosion resistance compared to the substrate. After Hybrid-NCT treatment, the anodic pwlarization curved is shifted towards lower current density which explain that the corrosion rate was decreased and the polarization current measurement gave 0.00003 mA/ cm and demonstrate an improvement in corrosion resistance as compared to that untreated and individually nitrided and carburized steel, while passivation current of NCT is the lowest followed by CT, NT and untreated respectively. This trend also similar to the maximum potential passivation behaviour since the... 

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