10. Not keeping safety programs and routine audits active to avoid accidents. Idle plants with small crews operating at a very relaxed tempo can be dangerous places.
A useful analogy in developing a strategy is to consider how plants fend off fire hazards by eliminating one of the fire triangle’s three elements — i.e., heat, fuel source and oxygen. The three corresponding elements for age-related deterioration are a driving force such as galvanic action, a conducting medium or electrolyte, and oxygen. The fundamental approach to stopping or slowing deterioration is to remove one or more of the three.
In simple terms we aim to do the following:
• separate dissimilar metals;
• protect surfaces that could be attacked — with a covering, even if only a few molecules thick;
• dry out or remove the conducting medium (electrolyte — air or gas) (Corrosion can’t occur when parts are stored in environments with relative humidity below 40%); and
• eliminate any oxygen and sources of chemical or biological attack.
Materials and equipment we can use are:
Liquid protective waxes and liquid polyvinyl chloride (PVC) coatings. These can be sprayed onto any clean dry surface to protect it. Light waxes are chosen for surfaces where a subsequent removal process such as high-pressure washing might damage the substrate. PVC will form a tough, flexible and waterproof skin that will withstand temperature extremes, thermal shock, differential substrate movement and impingement, even when sprayed onto webbing to form a cocoon.
Volatile phase/corrosion inhibitors (VPI/VCI). Such materials generate protective vapors even at room temperature. They come in a number of convenient forms, including time-release vaporizers, sprays, plastic bags and films, powders, oil additives (see VSI below) and coatings. They are adsorbed (just a few molecules thick) onto metallic surfaces of equipment and can prevent corrosion for up to two years.
They actually have surprisingly low vapor pressures and are solid or liquid at room temperature. While there are many chemical compounds in use, the most common form of VPI is a salt of an amine (e.g., the carbonate salt of dicyclohexylamine) and a weak acid. Research has shown that they work by disassociation of the amine and the acid; the two volatile components then recombine on the metal surfaces.
While most VCI are environmentally friendly and create no safety hazards for employees, some are suspected of being harmful. Most contain no toxic substances such as nitrates, chromates or volatile organic compounds (VOC). (Note: products containing VOC shouldn’t be used in combination with a desiccant.)
Vapor space inhibitors (VSI). These concentrates can be added to lubricating oil systems (internal combustion engines, etc.) when equipment isn’t going to be completely filled. They essentially boil at ambient temperatures to exclude air, leaving an oily residue.
Heat-shrinkable desiccant plastic films. Such films containing desiccants are ideal for enclosing individual machines that have been cleaned and dried.
VCI-covered polyethylene films. These suit wrapping individual smaller components.
Chemical oxygen scavengers. These compounds are added to fresh water used to displace more-corrosive liquid in systems that can’t be effectively cleaned or dried out.
Chemical inhibitors. Incorporated into liquids, they remove unwanted products while preferentially inhibiting their attack on the body of the container. (Anti-freeze sometimes used in mothballing contains them.)
Desiccants. Numerous solids can absorb water from gases (air) or liquids.
Biocides. Such materials prevent microbial growths in water and fuels like gasoline and diesel fuel.
Sacrificial anodes. Made of materials such as magnesium or aluminum, they are used in tanks that can’t be drained of their contents.