Tank Surface Damage: Pitting, Corrosion, & Deformation

Introduction

Surface damage on a tank can be unsightly, but more importantly, it can stop your equipment from operating at its best.  Understanding the types of surface damage can help you control them better. Here is an overview of the 3 most common types.

Pitting

This is a localized type of corrosion in the form of cavities.  These pits come in many forms, including narrow and deep, wide and shallow, and elliptical. Sometimes, the pitting will undercut and begin to run parallel to the surface.  The most common cause of pitting is chloride ions (which can be found in salts and bleach), which damage the passive film of stainless steel.  Pitting can be prevented by controlling your workspace's pH, chloride concentration, and temperature.

Corrosion

Several factors can cause corrosion, including the characteristics of the media flowing through the system and the environment in which the system is located.  Similar to pitting, which is a type of corrosion, the pH, chemical makeup, temperature, and velocity of the media are all contributing factors to the corrosion rate.  Environmentally, air temperature and humidity levels will also contribute to corrosion.

The most common types of corrosion that occurs are:

• General corrosion - an entire surface begins to deteriorate, and the material will continue to weaken until it reaches the point of failure.
• Localized corrosion includes pitting (see pitting), crevice corrosion, typically found under gaskets and clamps, and filiform corrosion, which occurs under painted surfaces, starting at small defects in the coating and spreading under the surface.

Deformation

The material that makes up your tank can distort and bend while it is being used in a different shape than the original structure. This is known as deformation.  For example, when vacuuming your tank, the sides may flex in and out as the pressure changes.