Because stainless steel is easily cleaned and does not rust under normal conditions, it is the preferred metal for food processing machines used in wet environments. A little-known fact about stainless steel is that it is prone to unique types of corrosion when the metal is exposed to a specific set of environmental factors. These corrosion-producing conditions can exist during the operation of stainless-steel batch retorts and are not easily realized or recognized. Unless steps are taken to identify, reduce, or eliminate the causes of corrosion, batch retorts may have their service lives cut short, resulting in production disruptions and the need for early replacement of affected vessels.
Modern retorts and their internal wetted parts are commonly made from 300-series stainless steel. Understanding the root causes of corrosion in stainless-steel retorts can be complex; however, there is a common theme in how most of the problems start. The standard retort has an internal environment of high temperature and pressure created by steam, water, and oxygen—all of which are normal operating conditions for a retort. When excessive harmful chloride ions are present—inside or even on the outside of the retort—regardless of how they were introduced, the ions act as a catalyst for starting and accelerating corrosion in stainless steel during the retort cycle.