AWS PRGC PDF

Introduction

Corrosion, resulting in the severe degradation of materials, is one of the most expensive engineering problems in our industrial society; estimates have been made that the annual cost of corrosion in the U.S. exceeds 100 billion dollars. As you might imagine with the financial stake so high, manufacturers and end users expend significant amounts of engineering time and money avoiding, protecting against, or repairing damages from corrosion. A very large majority of industry must, or should, consider the environmental effects on its products or equipment and guard against premature failure due to corrosion.

Most of us are aware of a very common corrosion problem, corrosion of our automobiles. If you have lived near a coastal water area, or in the snow belt areas that keep the roads clear of ice and snow by spreading salt on the highways each winter, you are probably familiar with the rusted-out car bodies that occur quite quickly when exposed to the moist salt-air environment. Our automakers spend considerable effort to protect their products from frame and body corrosion by these severe environments, but quite often, Mother Nature wins the battle.

Other common examples of items needing corrosion protection include bridges, electrical connections in appliances and electronic devices, chemical processing plants, water pipes, hot water heaters, structural steel, welds; the list is endless. Suffice it to say that corrosion is a pervasive problem and industry must first understand corrosion before they can take steps to solve the unique problems it presents.

Welded structures are often subjected to corroding environments; in some cases, the weld and base metal corrode uniformly at the same rate. In other cases, the results are accelerated corrosion of the weld compared to the base metal, or the base metal may corrode at a much faster rate leaving the weld metal relatively intact. Welding, and its associated heat input, can also contribute to other corrosion problems; these will be discussed in greater detail later.

A logical starting point for dealing with corrosion of welds is to define corrosion and then list the various types of corrosion that can occur, with examples. Definitions used are those developed by Dr. Mars Fontana and/or NACE. There are many different forms of corrosion recognized that include the various corrosion mechanisms, and the most common are defined and discussed below. The next logical step after defining the various forms of corrosion is to then develop methods for avoidance of each and these are also listed following each definition. Several case histories are also noted to demonstrate actual, practical solutions.