Deterioration due to corrosion of materials, parts and components during transportation and temporary storage during interstage manufacturing is a persistent problem in the industry. It causes direct material losses as well as indirect losses. Due to temporary corrosion, time is diverted to handling complaints and customer loyalty is potentially lost. Delayed deliveries due to reworking of corroded components translate to avoidable losses. Therefore, planning for corrosion protection during temporary storage and transportation is a logical strategy to minimize these losses.
The Importance of Temporary Corrosion Protection
There are many industrial applications where temporary corrosion protection during transit and storage is critical for the final processing and end use of parts and components. If semi-finished or machined components are left unprotected, or roller bearings are not coated with a rust inhibitor during storage, or internal combustion engines are transported and stored without corrosion prevention, then they may deteriorate or become damaged due to corrosion. Any subsequent rust removal will not restore a component to its original quality and geometrical accuracy. The best way to protect such materials and components is to apply suitable coatings for temporary corrosion protection. (For a thorough background on paints and coatings, see The Composition of a Paint Coating.)
Permanent corrosion protection cannot be used for temporary applications because it must be removed before further processing or assembly. Because corrosion can appear within hours or days a temporary corrosion prevention method must be implemented.
Materials and Components Requiring Temporary Corrosion Protection
During transportation and storage, fasteners (e.g., wedges, springs, bolts, nuts, washers, screws) and associated tools require protection from corrosive chemical reactions on their surfaces. (Learn more about fasteners in High Pressure Fastener Coating Practices Under Fire.) Equipment and parts requiring corrosion protection include:
- Machinery spares, turbine blades, generators and parts
- Shafts, trunnions, turbine component parts
- Bores, threaded holes, axles, couplings, gear boxes, fittings, seals
- Measuring instruments, precision devices and controllers
- Automobile components and sub-assemblies
- Mechanical parts, ball and roller bearings
- Tools and component parts
- Steel pipes, tubes, strips, sections, structural members
- Aluminum sections, alloy sections and strips
- Electrical sub-assemblies and parts, electrical components, coils
- Spare parts
- Agricultural machine parts
- Materials used for ship building
General Care for Preventing Corrosive Deterioration
Desiccants to control air humidity are used to prevent corrosive deterioration during storage and transport. Silica gels and molecular sieves are occasionally used as desiccants to absorb excess air moisture. Sealed films of high pressure polyethylene and special waterproof and airproof packaging systems are used for critical applications. Hygroscopic fillers can also be used in the packaging.
Moisture-barrier bags (foil bags) are used for packing important components, as well as critical parts and instruments.
These devices provide protection against corrosion due to moisture, humidity, salt, acidic contaminants and oxygen. Components with barrier protection are bagged and sealed in a benign environment to ensure complete protection during marine transportation.
The natural draining of condensed water is ensured by placing the materials in an inclined position during transport and storage. A horizontal (level) positioning that can cause water and dirt to accumulate should be avoided. The use of undrained grooves should be minimized.
Creating a Temporary Barrier Film
The following methods are useful to provide temporary corrosion protection:
Brush and Smear Methods
For waterborne fluids, brushes made of polyester or nylon are used. For very thick primers, brushes with round bristles are used. For components with hard to reach surfaces, brushes with angled shafts are helpful. For varnishes, rectangular brushes are useful.
The brush method is time-consuming and inefficient for large work volumes. The smear method is another manual method and is used for locations where other methods are not feasible.
Various types of spray systems are available. The spray method is used for small auto parts and auto body components. The spray technique has the advantages of providing a uniform application and low coating wastage. Automatic spray methods are adopted for a larger volume of components.
Coating Selection for Temporary Corrosion Protection
The selection of an appropriate coating is based on:
- The metal to be protected and the period of protection
- The type of transport environment (marine or road) and the ambient conditions
- A feasible application method and removal requirements for subsequent processing
- Regulatory requirements regarding safety, fire risk and waste disposal
Parts stored in a warm indoor room may be coated with grease or a thin oil film.
Parts stored outdoors without external wrapping may be coated with a thicker hard film via hot dip or vapor deposition.
For wet components and parts, an emulsifiable fluid oil coat or water displacement type coat may be chosen.
Small components (e.g., fasteners) may be coated with an oil film coating and then seal-wrapped.
Assembled parts with a rubber seal or packing may use vegetable-based grease.
Some Examples of Temporary Corrosion Protective Coatings
Acrylic insulated varnish is appropriate for electric motor windings, coils, collector rings, bus-bars, switchgear parts, components, motor end shields and armatures. Acrylic copolymers are used for new vehicles.
Zinc silicate, which has a short drying time, is used during shipbuilding as a shop-primer for steel that is temporarily stored for subsequent welding. (For more information about zinc silicate, read IOZ Coating Surface Preparation is Easier than You Think.) Epoxy iron oxide is sometimes used, but requires a heavy film thickness.
General corrosion and rust preventives include aqueous fluids, petroleum oil and emulsions, mineral or vegetable oil grease, wax, and volatile rust inhibitors. Petroleum-based oils and greases act as temporary corrosion preventives. They do not stain, but may be incompatible with seals and packings. A low flash point implies the risk of a fire hazard. Some producers offer high flash point products. The brush method, smearing and the dip method is suitable for oil-based preventives. These are easily removed by solvent cleaning.
Expert fluids with corrosion inhibitors that produce tenacious thin films are suitable for ball and roller bearings, small steel strips and sheets.
Cum running-in preservation oils (with anti-corrosion additives) are used as the first fill for crankcases of internal combustion engines. Multigrade oils are also suitable for tractor engines and other engines for corrosion protection.
Process oils used for perforating, drawing, forming and punching operations also provide corrosion protection. Synthetic or mineral oils with additives are used as process oils.
Biodegradable (vegetable as well as biological) metalworking, metal forming and hydraulic fluids under development have exceptional corrosion protection capabilities. They are non-toxic, nonflammable, eco-friendly and do not require permits for disposal. They have lower evaporation rates, are safe for confined spaces, and are suitable for eco-sensitive applications such as food processing and agriculture.
Molten wax films are used in transportation and storage for temporary corrosion protection as well as preventive application. They are considered superior providers of rust protection because a protective film can be formed in inaccessible surfaces such as in engine compartments and car frames.
Aqueous fluids provide a barrier that protects metal surfaces for three to six months. These chemicals can be added to metal cleaning baths. The film need not be removed during subsequent heat treatment. These are used for interstages of manufacturing storage. However, the chosen protective must be checked for compatibility with seals and packings, adhesives and nonferrous components.
Cost, Safety and Health
Allowing temporary corrosion and rust on materials during interstage processes and transportation is costly due to the direct and indirect costs involved in rework and warranty damages. When carefully chosen, preventive solutions are cost effective for both application and subsequent removal.
Risk factors related to volatile corrosion inhibitors (VCI), mineral oils and additives must be studied. Flash points of the fluids could vary considerably. In case of a fire, water is not to be used. For small fires, carbon dioxide, foam, sand or dry earth may be used.
For confined spaces, authorized specialists must be consulted. Ensure compliance with personal protective equipment (PPE) and firefighting regulations.
The combustion products of coatings can be highly toxic. Hence exposure control must be planned for in advance. Precautions should be taken to avoid eye contact and skin exposure with proper PPE suitable for the application method used. Respiratory protection might be needed in a few cases.
Proactively prevent leaks that may lead to drains becoming filled or blocked. Avoid PVC containers or linings. Carefully study the coating's material safety data sheet (MSDS).
Coatings generally are not biodegradable, and must be disposed of per local regulations (certified contractors may be engaged). Alternative coatings with vegetable oil compounding for easy biodegradability are under development.
Preventing temporary corrosion is an essential strategy for winning the battle against material loss due to the electrochemical and chemical deterioration of metals during manufacturing stages, transportation and transit storage. The choice of an appropriate preventive coating should be done while keeping the subsequent stages of manufacturing, assembly or end use in mind. Temporary corrosion prevention is a necessary insurance against future losses.