Abstract

A new large Type 316L Stainless Steel hot potable water system had unexplained massive failures. To find the source of the corrosion failure the following causes were investigated; biological effects, metallurgical variations, chemical influences, process influence, lab testing and field testing. This program consisted of consultants, vendors and an on-site testing program using actual conditions to determine the cause of the corrosion. This paper covers the investigation, summarizes the findings and suspected causes and proposes a mechanism to explain why the stainless steel system failed as it did in such a seemingly benign environment.

INTRODUCTION

A new 316L (S31603) stainless steel hot water system, after being in operation for 6 weeks, started to leak at a welded joint. Soon there were three leaks and more followed quickly. Inside the pipe, at the leaks were corrosion tubercles on both the welds and parent metal. Examination of the area under the tubercles showed the characteristic scallop shell pattern, circumstantial evidence pointing to microbiologically induced corrosion (MIC). One analysis of the corrosion product showed abnormally high manganese in one location. This didn’t make sence. The water was both softened and chlorinated and the water analysis showed essentially no manganese. Then the tanks began to leak.

To confound the problem even more, the operation and the materials of construction of this brewery system were essentially the same or of a higher corrosion resistant material then used in its sister breweries. The water chemistry was essentially the same at all of the facilities. The problem had to be unique to this location.

Experts were called in from many areas, including major Universities, Independent Consultants in both water chemistry and metallurgy, Chemical Suppliers, Specialists in MIC and a Stainless Steel Supplier.

THE SETTING

Clean, high quality water is essential to all modern breweries. And this Miller brewery is one of the newest and most modern in the world. The water used in this ultra modern brewery must meet the highest quality standards. The water used at this facility is cold lime softened to remove the iron, manganese, magnesium and most of the calcium. As it leaves the flocculation basin, it has a pH of about 10.5. It flows into the first carbonation basin where the pH is reduced with carbon dioxide. This has the dual effect of reducing the pH and the total calcium in the water. The pH is about 8.5 as it exits the basin. Then the water is given a second carbon dioxide treatment to reduce the pH to 6.7. In a brewery, carbon dioxide is plentiful, so it is a convenient reagent. From here the water goes to the carbon filters, it is chlorinated. For the system described in this paper the water is next heated to 185?F (85? C). This water is used for system cleaning. A schematic of the piping system is presented in figure 1.

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