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Electrochemical noise (EN) probes were deployed in the carbon steel continuous kraft digester at Spring Grove at four locations and at one location in the bottom cone of the associated flash tank for a second consecutive year of a corrosion study. The probes contained dual electrodes of 309LSi stainless steel overlay--representing a field repair material applied to a portion of the vessel--and dual electrodes of 312 stainless steel overlay. Current and potential noise, the temperature at each probe location, and the value of 23 process parameters (flow rates, liquor chemistry, etc.) were again monitored continuously for a period of almost one year. Historical vessel inspection data and post-test evaluation of the probe components were used to assess/compare EN corrosion activity with physical changes in wall thickness and corrosion patterns on the digester shell. In addition, attempts were made to correlate EN activity from each electrode type with process parameters. The results indicate the corrosion conditions aggressive to mild steel persist within the digester, as post-test inspection of the vessel revealed localized corrosion of mild steel in locations previously free of attack. Further, there was evidence that the depth of localized attack of exposed steel had increased in some locations. Nevertheless, the stainless steel overlay in the digester was essentially immune to corrosion, as evidenced by retained surface relief and heat tint associated with the original deposition process. The 309LSi electrodes also appeared visually pristine, and post-exposure metallographic examination of the 309LSi electrode materials revealed no attack. The 312 electrode materials were similar in appearance, but exhibited very minor interdendritic attack over the exposed surface. The silver electrodes in the probes were consumed (to Ag{sub 2}S) to variable degree over the course of the exposure indicating a useful life of not more than a year in digester service in this vessel. Since the stainless steel overlay electrodes were immune to corrosion during the exposure, the current and potential noise activity on these probes is likely related to redox processes on the electrode surfaces. Analysis of this activity as a function of position (and year) in the vessel suggests that redox chemistry/conditions vary by a significant amount on a consistent basis--even on opposite sides of the vessel at the same elevation--and that these differences are not identified by process parameters tracked at the mill. These variable environmental conditions (flow, temperature, liquor chemistry) appear to have little effect on stainless steel overlays under evaluation, but apparently can be quite corrosive to steel as determined in the initial study in the digester at Spring Grove. With the exception of start-up and shutdown activity, including brief upsets for ''hanging columns'' or brief maintenance periods, no regular correlation was observed between tracked process variables and EN activity on any of the probes. In combination with the variable redox activity, this result suggests that the liquor sampling and flow data compiled at the mill do not represent the corrosion conditions in the vessel particularly well

Correlation of Process Data and Electrocheical Noise to Assess Kraft Digester Corrosion: Second Year at Spring Grove