Monitoring The Effect Of Pulp And Paper Mill Process Changes On The Corrosion Resistance Of 316 And 316L Austenitic Stainless Steels

This paper presents the results of corrosion studies in two pulp and paper mills employing two corrosion monitoring techniques: (1) linear polarization resistance (LPR) and, (2) electrochemical noise (EN). These online electrochemical corrosion monitoring techniques were used to evaluate the change in corrosion rate and pitting resistance of Type 316 and 316L stainless steels, as a direct result of chemical change of white water process streams.

The list of industries using on-line corrosion monitoring techniques has grown since the oil production and refinery industries first applications over thirty years ago. The oil and gas industry has utilized various electrochemical corrosion monitoring techniques more than other industries in the recent past.^(1-6) The chemical process industry and other industries are starting to use corrosion monitoring techniques to evaluate the corrosion resistance of materials in support of life expectancy and performance studies involving process equipment.^(7-10) Recently, electrochemical on-line corrosion monitoring techniques have been employed in the pulp and paper industry to evaluate the performance of Kraft digesters, ^(1-13) kraft clarifiers,⁽¹⁴⁾ and a spent sulfite fueled boiler.⁽¹⁵⁾The results of the recent work in pulp mill process streams produced good correlation between material corrosion rates and process stream changes. Corrosion monitoring has been shown to be a good tool for assessing the condition of pulp mill equipment.

On-line corrosion monitoring can provide a continuous measure of corrosion rate, employing electrochemical techniques, or it can utilize periodic measures of corrosion. Examples of periodic measures include: weight (mass)-loss coupons, electrical resistance (ER), and non-destructive evaluations (NDE). Examples of electrochemical techniques that can be used for continuous monitoring and process control include: linear polarization resistance (LPR), galvanic currents, electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS). A recent paper describes these various electrochemical techniques including their benefits and their integration with process control. ⁽¹⁶⁾LPR and EN are the two on-line continuous corrosion monitoring techniques employed in the studies discussed in this paper. The application of LPR for monitoring corrosion and interacting with data acquisition systems is widespread. Typical applications nave included various oil field equipment, ^{(17, 18)} pipelines for inhibitor evaluation,^{(19, 20)} chemical plant components,^{(21, 22)} pulp mill equipment transporting liquors,⁽²³⁾ and water treatment plant equipment.⁽²⁴⁾ Electrochemical noise (EN), or electrochemical potential transient technique, has been reviewed by several investigators.^(25-31) The EN methodology involves the measurement of electrochemical events (i.e. transients or noise) produced by the corrosion process. This is typically accomplished by (1) measuring potential noise which results from the potential difference developed between two identical electrodes or, (2) measuring current noise which results from the current fluctuations of two identical electrodes shorted together, or (3) measuring both potential and current noise between identical electrodes simultaneously.

Corrosion monitoring studies were conducted in two pulp and paper mills to assess the impact of process changes on the pitting resistance of pulp mill and paper machine stainless steel components. Test procedures employed are contained in ASTM Standar