How to select a rust inhibitor
October 10, 2006
Rust, wear, and dirt cost tube fabricators and producers millions of dollars annually, and they can be the bane of tube processes. Analyzing the criteria for selecting the lubricant, cleaner, and rust preventative can help provide maximum protection.
One of the most common problems in the manufacturing industry is rust. This four-letter word costs manufacturers millions of dollars annually. The good news is that with careful consideration, you can eliminate costly waste caused by rust. This can be done by understanding the different types of rust inhibitors, or preventives, and their properties, as well as specific details on how these inhibitors should be applied.
While a variety of rust inhibitors are available, five chemical rust preventives are commonly used:
Synthetic compounds are nonpetroleum products that are nontoxic, nonhazardous, and environmentally acceptable. They normally are used in the final stage of a spray washer at 5 percent to 10 percent concentration in water.
Straight oils normally are applied at the steel mill as a mill or slushing oil. They provide short-term protection and can be press-applied.
Soluble oils are water-soluble. They are used in spray washers and immersion baths at 5 percent to 25 percent concentration.
Solvent compounds contain polar- or barrier-type dissolved solids that remain on the metal surface after the solvent carrier has evaporated. These rust preventives are available with a range of flash points, depending on safety regulations, and may be applied by hand spray, spray booth, brush, or parts tumbler.
Water-displacing compounds contain polar wetting agents that have a strong affinity for metal surfaces. They quickly displace water off parts and to the bottom of an immersion tank.
When you are selecting a rust preventive, you must be aware of several critical factors to provide maximum part protection. By first answering a series of questions, you will be in the best position to select rust inhibitors that are well-suited for your application.
1. How long will parts be stored, and in what environment? Indoors? Outdoors? Sealed packaging? The protective film left by the rust preventive is crucial in parts protection. In addition, the environment it is stored or transported in affects the protection. If the plant is not climatically controlled, you need to assume that condensation will accumulate on parts as the temperature fluctuates. In this instance, a protective film that is not easily removed by condensation would be required for periods of protection longer than six to eight weeks.
If the parts are stored in a high forklift-traffic area, the rust preventive must account for the corrosive exhaust gases the parts will be exposed to. Most inhibitors can protect against this type of rust. However, in high-traffic areas with little ventilation, a heavier-duty preventive may be needed.
Outdoor storage always presents a challenge because the environment varies. Inhibitors composed of solvents and oils work well for parts that are stored outdoors short-term—for three months or less—or for longer than three months if the parts are covered with a tarp or overhangs. However, most preventives meant for long-term outdoor storage are pastes or heavy oil types.
2. How will the parts be shipped? Transit conditions are important. If the parts are coated in Texas and shipped to Maine, they will experience varying temperatures and be susceptible to moisture accumulation. The inhibitor must not be easily washed off by condensation. A variety of solvent- and oil-based preventives satisfy this requirement. Incidentally, just because the inhibitor is resistant to removal by condensation does not mean that it will be resistant to removal in a degreasing operation or alkaline washer.
Will the part be shipped overseas? Transcontinental shipping has increased steadily in recent years. As many companies source manufacturing operations overseas, many parts that are shipped to the U.S. may be exposed to salt air, moisture, and varying environments. Several companies have outsourced manufacturing operations, such as machining, only to have the parts return spotted, corroded, or heavily coated with rust. A water-displacing, semifirm, dry-to-the-touch preventive is best-suited for transcontinental shipments.
3. What type of film can be left on the parts? Can the film be oily? Dry? Heavy? Waxy? The type of film that can remain is important for postprocessing applications (see Figure 1).
Will people handle the parts? Sweat contains high levels of salt, which promotes rust. If the metal parts will be handled by humans without gloves, it is important that the rust preventive have additives blended into the formulation to neutralize the salts left from human fingers.
Many rust preventives have these additives as a standard component. However, often this is overlooked, resulting in rusty parts.
Will the parts be painted? If they require short-term protection, a light film solvent, oil-based preventive, or an easily removed water-based preventive is best, depending on how much time lapses between application of the rust preventive and the painting.
Will the preventive be in contact with rubber seals or O-rings? Some oil-based preventives may cause swelling on seals. You can conduct a simple lab test before you use a preventive to find out if it will affect seals or O-rings.
4. How will my customer clean the parts? Knowing this facilitates a smooth integration of the rust preventive and will ensure that any residual film will not negatively affect the cleaning process.
If your customer will clean the parts with solvents, then most oil- and solvent-based preventives can be used. A water-soluble synthetic may react with the solvent, leaving a slightly gummy residue, depending on the chemistry of the preventive. You can remove most synthetic water-soluble preventives with hot water or a mild detergent alkaline cleaner. Generally, alkaline cleaners can remove most preventives on the market.
5. Do I need to use the rust inhibitor on wet parts? If you are coating wet parts, you will require a water-displacing, penetrating preventive to ensure complete part coverage and protection. The preventive flushes out the water, leaving a protective film.
6. Are parts tightly nested? If they are closely nested, viscosity will play an important role, because tight nesting requires that the inhibitor have good penetrating qualities. If the preventive is too thick, not all parts will be coated, resulting in rust. Conversely, if the preventive is too thin, it may not coat the parts adequately.
7. What are the disposal considerations? Unless a company has on-site waste disposal, most oils and solvents usually cannot be discharged to a waste stream and, therefore, must be hauled off-site by an authorized waste disposer (see Figure 2).
8. Is mixing or maintenance acceptable, or do I need a ready-to-use product? This applies to water-based inhibitors or firm, waxy-type inhibitors.
9. Is adequate ventilation available? The application of many solvent inhibitors requires ventilation, such as fans or air scrubbers, to reduce operator exposure to fumes. Each preventive has a threshold limit value, reported on the MSDS. If adequate ventilation is not available, you must determine if the exposure limits are within the acceptable ranges.
Recent technology advances have prompted the development of dry-to-the-touch, durable, water-soluble products. In addition, in response to demand, a combination cleaner and rust preventive has been developed. Such products do not contain volatile organic compounds (VOCs) and are more environmentally friendly than their predecessors. While these compounds will not fully replace solvent- or oil-based preventives, they are capable of providing sufficient short-term protection.