Corrosion control technologies are advancing to meet the challenges of globalization, regulations and changes in metallurgy and metal processing operations. With more robust defenses to mount against this scourge, the resulting benefits can be seen in everything from metalworking fluids to industrial, automotive and marine lubricants and greases.
Its difficult to overstate the value of corrosion control. Joshua E. Jackson, Ph.D., CEO of G2MT Laboratories in Houston, Texas, projected that combined direct and indirect costs of corrosion in the U.S. this year alone will exceed one trillion dollars, or 6.2 percent of estimated national gross domestic product. Global direct cost of corrosion was $2.5 trillion in 2013 – 3.4 percent of global GDP, according to the 2016 International Measures of Prevention, Application and Economics study issued by NACE International (formerly the National Association of Corrosion Engineers).
This is not a new battle. A landmark study initiated by NACE and mandated by the U.S. Congress estimated that direct corrosion costs were $276 billion in 1998 (3.1 percent of U.S. GDP), and other studies tallied comparable indirect costs, Jackson explained.
Change Is Constant
Several challenges are changing the way industry approaches the problem of corrosion. Ted McClure, team leader for technical resources at Sea-Land Chemical Co. in Westlake, Ohio, told LubesnGreases that Global competition is driving higher productivity and less downtime in manufacturing processes. These production changes put more stress on lubricants and affect selection of base stocks and additives. For example, he said, Corrosion inhibitors such as carboxylate salts, sulfonates, succinates and amides must now perform in metalworking fluids that resist foaming and tolerate a greater range of machining conditions. Faster machine speeds can cause higher operating temperatures, and formulations must be more thermally stable and less volatile than in the past.
Globalization and global competition are affecting supply chains, causing unexpected changes in chemical sourcing, McClure commented. When corporations spin off or acquire companies or respond to competitive pressures, they may streamline their operations and product offerings. These activities can affect availability of lube additives or basic chemicals used to manufacture additives such as corrosion inhibitors.
McClure pointed to the example of Koch Industries subsidiary Invista, which has discontinued making Corfree M-1 corrosion inhibitor. Metalworking fluid manufacturers that had relied on the popular product are looking for new additives that can provide corrosion inhibition comparable to Corfree M-1 but do not increase foam levels or decrease sump life because of lower tolerance for hard water salts, he noted.
In general, McClure summarized, cost effectiveness is increasingly important due to continuing globalization and competition.
Chemical supply chains can also feel the impact of changes in regulations. For example, the U.S. Environmental Protection Agency banned certain types of chlorinated paraffins and may eliminate other varieties of CPs from U.S. markets. CPs are go-to extreme pressure additives for metalworking fluids used for machining titanium alloys, stainless steels and other metals under severe processing conditions. Chemical suppliers and metalworking fluid blenders are working to replace CPs with new chemistries, and may have to find different corrosion inhibitors that are compatible.
Regulations may have indirect effects, too. According to McClure, Automotive manufacturers are quickly adopting advanced high-strength steels, new aluminum alloys and magnesium in order to manufacture fleets that meet corporate average fuel economy standards. Formulators must select corrosion inhibitors and balance metalworking fluid formulations to avoid staining these metals. And forming operations generate much higher part and die temperatures with new high-strength steels than traditional steel alloys. Thus, corrosion inhibitors must be more thermally stable and less volatile than in the past.
Shipping and production must also be included in the global business equation. McClure explained, Formed parts are often shipped to remote assembly plants. On one hand, it is necessary to prevent corrosion during storage and transport, but corrosion inhibitors must not interfere with production of finished products at assembly plants.
Lubrizols metal protection research manager, Britt Minch, Ph.D., told LubesnGreases that several chemical options exist for protecting metal surfaces from corrosion during storage and transport. Corrosion inhibitors are surface-active chemicals that typically provide short-term protection to metal. There are soluble and volatile varieties. Solution-phase corrosion inhibitors are added to fluids such as lubricants that come in contact with metal surfaces. Volatile corrosion inhibitors are released gradually from film or powder inside closed environments such as packaging.
In contrast, Minch noted, Rust preventatives are temporary protective coatings used on metal surfaces during storage. They provide more robust protection than corrosion inhibitors in lubes, but less protection than industrial maintenance paints. Rust preventatives are formulated for performance ranging from light to heavy duty.
Proving Ones Metal
Test methods differ for corrosion inhibitors versus rust preventatives, as Minch explained: Water-based corrosion inhibitors for metalworking fluids are often tested with the IP287 or IP125 metal chip tests in the lab. These tests from the London-based Institute of Petroleum use cast iron chips. Immersion tests such as ASTM Internationals D130, which uses copper chips in testing oil-based fluids, are also used. Fluid suppliers often choose test conditions that model end user applications, and field tests confirm fluid performance in real-world environments.
End-users expect rust preventatives to provide more effective protection than corrosion inhibitors, Minch continued, and so-called accelerated laboratory tests are used to evaluate performance.
A key test for rust preventatives is the ASTM B117 Salt Spray (Fog) Test, in which coated metal panels are placed in a chamber similar to a dishwasher at 35 degrees Celsius and sprayed continuously with a 5 percent sodium chloride (NaCl) solution. Rust will completely cover unprotected steel panels within 15 minutes, reported Minch. Salt spray data (observations of rust) are used as a guideline for product selection, although sometimes this test is not representative of the end-use environment.
ASTM D1748 is a less severe test option, which exposes metal panels in a cabinet to 100 percent relative humidity at 49 C. After 24 hours, rust is visible on uncoated steel.
Exterior exposure tests are utilized to test rust preventative coatings in outdoor environments. One such test, ASTM D1014, calls for coated panels to be placed on an outdoor exposure rack facing due south with panels at a 45-degree angle from vertical. This ASTM standard method is fine for heavy duty rust preventatives and paints, Minch explained, but it is too severe for light duty temporary coatings. Instead, the test can be modified to evaluate light duty rust preventatives, using a cover or shelter to partially protect test panels from the elements. In either case, exterior exposure test results are specific to the geographic region and time-frame of exposure. And it is necessary to evaluate panels for staining, blistering and corrosion until temporary coatings fail, which can take years for heavy duty rust preventatives.
Whether testing corrosion inhibitors or rust preventatives, a single test does not predict performance, Minch cautioned. In our experience, a combination of tests is the best approach to learning how corrosion inhibitors and rust preventatives will perform over their service life in the field. Whenever possible, testing should be done under conditions that are similar to the anticipated service environment.
Advances in Prevention
At a recent meeting of the Philadelphia section of the Society of Tribologists and Lubrication Engineers, Bill Kingston, technical marketing manager for rust preventatives at King Industries in Norwalk, Connecticut, revealed some advances in rust prevention technologies. While temporary coatings traditionally have been formulated in solvents or oil-solvent blends, Kingston described work that is underway to develop additives that can be used to formulate temporary coatings in water or emulsions.
For example, an oil-in-water emulsion contains small droplets of oil and oil-soluble additives suspended in water. Surfactant molecules form a layer around each droplet, or micelle. These emulsions may appear cloudy, but the oil and water do not separate (provided the formulation is stable).
Advantages of water-based temporary coatings include a lower risk of fire and less worker exposure to solvents. However, Kingston said, they tend to dry more slowly than solvent-based coatings. Emulsions provide a means to use hydrophobic additives, which tend to provide long-term protection in aqueous formulations.
Kingston elaborated on a promising additive in a water-based coating (10 percent additive, 90 percent tap water) that passed 1,400 hours in the ASTM D1748 humidity test, and an emulsion (10 percent additive, 4 percent emulsifiers, 16 percent oil, 70 percent tap water) that passed 20 to 30 hours in the ASTM B117 salt spray test.
Another approach to corrosion control involves removing water from in-service lubricants.
Sudip Majumdar, Ph.D., chief technology officer, and Evan Sohodski, development engineer at Compact Membrane Systems in Wilmington, Delaware, noted that there are three states or conditions for water in lubricating oil: free water (large droplets or layers of water that separate from oil), emulsified water (droplets suspended in oil) and dissolved water (trace levels of water molecules soluble in oil). Majumdar cited a 1977 study by R.E. Cantley of Timken measuring the effects of water dissolved in lubes, which found that reducing dissolved water from 400 to 25 ppm in oil increased relative bearing life five-fold.
Water is responsible for corroding equipment. As you remove water, there are direct effects: less corrosion and longer equipment life, Majumdar observed. And there are other effects for lubricants formulated from certain base oils. For example, water can cause organic ester and phosphate ester base oils to degrade. Water hydrolyzes esters – that is, it reacts with esters to form alcohols and acids, leading to increased corrosivity. The reaction is autocatalytic. This is particularly important at nuclear power plants, because even stainless steel alloys are vulnerable to corrosion by acids.
Sohodski noted that several different types of equipment are used to remove moisture from lubes. Centrifuges and absorbent filters remove free and emulsified water from oil but have no effect on dissolved water. Vacuum dehydrators can remove dissolved water, but they are bulky and may be prone to flooding and foaming. However, he explained, our equipment removes free, emulsified and dissolved water from lubes in real time. We circulate oil around tubes that are made of an advanced membrane material developed by DuPont. A vacuum inside each tube pulls water molecules from the oil into membrane pores and then carries the water along the tube and releases it into the atmosphere.
U.S. EPA Vessel General Permit regulations now require use of environmentally acceptable lubricants in all shipboard machinery and equipment where discharge of oils to surrounding waters is likely to occur, Sohodski continued. But EALs tend to be hydrophilic and dissolve relatively large quantities of water. Removing dissolved water in real time is critical for ocean-going vessels in order to extend engine bearing life. One answer is to use portable, compact dehydration units, based on filter carts, that fit in even small spaces on ships. Sohodski said the membrane dehydrators are also used in wind turbine gear boxes and other applications that are difficult to access, where the service life of mechanical equipment and lubricants is a priority.
Mary Moon, Ph.D., is a physical chemist with hands-on R&D and management experience in the lubricating oil and grease and specialty chemicals industries. She volunteers as treasurer of the Philadelphia section of STLE. Contact her at firstname.lastname@example.org or (267) 567-7234.
The Spillover Effect
Changes in the automotive industry have influenced the fight against corrosion in unexpected ways – such as the base oil options for making industrial lubes.
Evolving passenger car motor oil and heavy-duty diesel engine oil specifications call for new, lower volatility formulations based on low viscosity, low volatility base stocks, explained Sea-Land Chemicals Ted McClure. As a result, formulators are seeking corrosion inhibitors that are soluble and perform well in a range of base stocks, from highly refined API Groups II and III to PAO and synthetic complex and polymeric esters. Of course, corrosion inhibitors themselves must be low VOC materials.
The crankcase oil market affects base stock availability for industrial lube applications, so there is a spillover effect, McClure pointed out. In metalworking fluids and other lubricants, this means corrosion inhibitors must be effective when formulated in a wider variety of base stocks than in the past. The choice of base stock can affect additive response, including corrosion prevention efficacy. And formulators prefer corrosion inhibitors that dont interfere with other additives. – Mary Moon