Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation |
 It takes doing a metallurgical analysis only once to prove that what you see with the naked eye isn't necessarily what you get.
During the '97 spring outage, an internal inspection was made on a large fired dryer. The dome, which was made out of 309 stainless steel, was severely discolored with green tones. Further inspection from on top revealed that the dome was wafer thin in several regions, and these regions had buckled and redistributed the load. The dome was nine years old. A total of 13 metal samples were cut out from the dome. Seven of those samples were cut out near the central portion of the dome where the burner was located. These seven samples were thin. The other six samples were cut along the outer perimeter of the dome. These samples still maintained their original thickness and were not corroded. It was believed that this region could be salvaged. Therefore to some people, it made no sense to waste time analyzing something that to the naked eye looked bad, and then ruin the section of the dome that looked good by cutting out samples. The metallurgical analysis was almost not approved because of this reasoning. A hard head and a deaf ear prevailed. |
 Although the six samples in the supposedly good looking region hadn't thinned like the other seven samples, metallurgical analysis revealed that all six samples had indeed changed in a way that was not discernible to the naked eye. The grains within the stainless steel had undergone a transformation, and had become "sensitized." Chromium carbides had precipitated out along the grain boundaries, leaving a chromium depleted region. The picture (above left) shows chromium carbide precipitates along the grain boundaries. This region was less resistant to corrosion attack. The picture to the right shows where corrosive attack followed the chromium depleted grain boundaries.
At first it was feared that this intergranular attack was some form of stress corrosion cracking, but the propensity of chromium carbides disproved that failure mode. There is a hyperlink to the laboratory that did the work. Just click on any of the names in the pictures. |
 The picture to the right is an excellent shot of the grain boundaries. You're looking at the individual grains themselves as the crack followed along the chromium depleted boundaries. An overall view of the corrosion mechanism in action can be seen in the picture to the lower left. That's a picture of the scale (top) corrosion products (middle) and the intergranular attack (bottom). The picture to the lower right is a close up of the region between the scale and the metal, and highlights the corrosion products at the immediate vicinity of the surface. The two pictures below are a good example of why scale should be removed. The corrosion mechanism is protected by the scale. The appearance of the surface to the naked eye, when the scale is removed, is crystalline.
To improve the life of the dome the metallurgy was upgraded to 347 stainless steel. This grade of stainless steel has additional elements which preferentially combine with the carbon before the chromium has a chance to combine with the carbon. Therefore, the chromium is left in the matrix to provide the required corrosion protection. This type of stainless steel is usually referred to as "stabilized." . |
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