Non-Destructive Evaluation Method to Measure the Degree of Sensitization
Authors: Koketsu Tomomi 
Published: February 2020

Austenitic stainless steels, such as SUS304, are known to be sensitized by welding or other high-temperature operations. Since sensitized steels are susceptible to stress corrosion cracking (SCC), it is important to detect the degree of sensitization (DOS) of the heat-affected zone (HAZ) in and around the weld zone, based on which to properly conduct repair work, change materials, and manage paint surfaces, etc, in ensuring the integrity of production plants and other facilities.

The  method of electrochemical potentiokinetic reactivation ratio measurement for stainless steels(1), is one of the techniques used to electrochemically assess the DOS.In the electrochemical potentiokinetic reactivation ratio measurement (EPR) test, the zone to be assessed is anodically polarized and passivated and then reactivated by performing a reverse sweep to determine the DOS based on the ratio of the activation current peak during the anode polarization to the reactivation current peak during the reverse sweep. The EPR method is characterized by its ability to quantitatively assess the DOS relatively easily and quickly especially for less sensitized specimens.

EPR tests are performed in laboratories to obtain basic information, based on which tests are also conducted on site (on-site method). A lab test is carried out by cutting out a specimen containing the test zone and placing it in a beaker. The on-site method, on the other hand, can be performed without cutting out a specimen, but instead by placing a small acrylic pipe directly over the test zone (weld HAZ) of actual equipment used in a plant to immerse the zone in a test solution.This EPR method can be used, for instance, to non-destructively expose the potential risk of SCC in welded pipes, which are undamaged but have been used in a coastal area for a long period of time. For this reason, the advantages of the EPR method are better utilized for on-site measurements. However, measurement errors tend to occur more with the on-site method, as it is carried out under conditions, some of which differ from those of the lab test.

To solve this problem, EPR tests were conducted using laboratory sensitized heat- treated SUS304 stainless steel specimens and actually welded SUS304 stainless steel materials to take measurements under varying conditions to determine the degree of influence of each error factor. As a result, for the following things, I found that receives the measured values greatly affected by the conditions change: The distance between the reference electrode and test surface;  The test temperature; Measurement position (distance from the welded metal and the shape of the test surface); The angle of the test surface (whether it is parallel or perpendicular to the ground); Measurements of specimens containing welded metal parts.

These causes of these influences are as follows: Included the elevated influence of IR-drop caused by solution resistance between the working and reference electrodes due to the small test cell diameter (Φ11mm); Vertical distribution of the DOS of the test zone; The deposition of eluted substances on the specimens; The deposition of eluted substances on the test zone; Contains a weld metal to test zone is let the value inaccurate in proportion to the percentage of the area of the welded part.

Based on the above, effective approaches to measuring the DOS more accurately were identified as follows: Maintain a specific distance (10mm) between the reference electrode and the test surface when taking measurements; Measure the temperatures of the test zones and make adjustments to the test results; Use preferably a rectangular test cell while giving ample consideration to the distance from the weld wire; Adjust measurement taken using the correction formulas established as a result of this experiment if the specimen is not placed parallel to the ground; Adjust measurement taken using the correction formulas established as a result of this experiment if contains a welded metal part.

(1) Japanese Industrial Standard(JIS) JIS G0580 (2003)