The food processing industry is one of the largest manufacturing industries in the U.S., accounting for approximately 14 percent of the total U.S. manufacturing output. A typical food processing plant uses equipment of varying ages, constructed of a myriad of materials, including carbon steel, aluminum, stainless steel and plastics. Corrosion - an attack on a material due to a chemical or electrochemical reaction with a surrounding medium - can be an enemy of many of those materials.
Typical food processing plant maintenance is a constant battle against the effects of corrosion on metals in the plant. Food quality requirements lead most plants to select stainless steel as a material of choice. Assuming that the stainless steel consumption and cost in the food processing industry is entirely attributed to corrosion, the total annual direct cost of corrosion is estimated at $2.1 billion.
Battling corrosion should start up front with proper material selection for the plant. Metals with high potential energy are called active and corrode more easily. Some metals, like gold and silver, can be found in nature in their pure metallic form. They require little added energy to change them into a useable form. As a result, the balance of energy is relatively stable and these metals corrode very slowly. Metals with low potential energy are called passive.
Metals can be ranked by their relative energy potentials. Think of this as a listing by the amount of energy that is required to convert them from their natural state to a metallic useable form. This listing is called the Electromotive Force Series (EMF).
Metals such as aluminum, iron, steel, chromium and titanium form a thin oxide film under oxidizing conditions. This oxidizing film increases the resistance of the metal to corrosion. The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a "passive" film that resists further "oxidation" or rusting.
The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, the metal has become "passivated" and the oxidation or "rusting" rate will slow down to less than 0.002-in per year (0.05-mm per year).
Unlike aluminum or silver, this passive film is invisible in stainless steel. It's created when oxygen combines with the chrome in the stainless to form chrome oxide, commonly called ceramic. This protective oxide or ceramic coating is common to most corrosion resistant materials.
One step to economically enhance corrosion resistance and prevent corrosion is the selection and use of a corrosion resistant coating material. Steel is so widely used that some form of coating protection must be used in order to increase the life expectancy of steel structures, piping or conduit. Because the anode, cathode and metallic path are quite often on the same piece of material, isolating the material from an electrolyte is the easiest method of prevention.
Metallic coatings provide a layer that changes the surface properties of the piece to those of the metal being applied. The piece becomes a composite material exhibiting properties that are generally not achievable by either material if used alone. The coatings provide a durable, corrosion resistant layer, and the core material provides the load bearing capability.
The most widely used metallic coating method for corrosion protection is galvanizing, which involves the application of metallic zinc to carbon steel for corrosion control purposes. Hot-dip galvanizing is the most common process, and as the name implies, it consists of dipping the steel member into a bath of molten zinc.
In applications where more severe or heavy corrosion conditions exist, the galvanized steel is often coated with paint or other polymer coatings, such as PVC, for additional corrosion protection. For example, Robroy polyvinyl-chloride (PVC) externally coated rigid steel conduit has been successfully used to protect sensitive wire and cable systems in extremely corrosive environments for several decades. However, proper surface preparation and a rigorous quality assurance program are necessary to achieve reliable coating protection.
When coating adhesion fails due to improper surface preparation, the underlying zinc becomes the primary corrosion protection. In this situation, only the galvanized coating protects the rigid steel conduit; the PVC or polyurethane coating is no longer an effective corrosion protection. In the case of adhesion failure, the galvanized zinc coating will probably have a shorter service life than plain galvanized rigid steel conduit.
If you have the opportunity to select an anti-corrosion material, ensure that you have investigated the material and coating as the most appropriate for the expected type of environment. In other words, do not use aluminum if mineral acids are going to be used; do not use stainless steels within a salt environment.
In the past, for instance, we often painted some of the metal housings on our lighting system to prevent corrosion. This seemed to be only a temporary solution because within a relatively short time, the units needed to be replaced and the peeling coatings were a source of food contamination on the production line. Now we know that selecting another metal material and coating for the lighting system could save us hundreds of thousands of dollars.
Another corrosion issue had to be confronted with a section of conduit within the plant. This section of conduit had become corroded and the wires were exposed. Two different metals having contact with each other caused the corrosion. The two metals are now properly protected so corrosion and the chance of having exposed electrical wires again are now reduced significantly.
We have switched from a short-term to a long-term view of corrosion that offers our plant a significant reduction in the amount of material and labor costs in the plant.
Pumps & Systems, July 2007
