Corrosion is a significant challenge for equipment operators across a wide swath of industry. This naturally occurring phenomenon damages metal surfaces in bearings, gear boxes and other devices and shortens the service life of machinery and lubricants. Formulating lubricants with effective and cost-efficient corrosion prevention additives, however, can present its own set of challenges.
In an interview with LubesnGreases, Maureen Hunter, Ph.D., and Bob Baker of King Industries in Norwalk, Connecticut, emphasized that corrosion protection is a priority in the lube industry. ASTM Internationals G193 (Standard Terminology and Acronyms Relating to Corrosion) defines corrosion as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. For example, general corrosion etches metal surfaces and reshapes parts, while localized corrosion initiates microscopic pits and pinholes.
Baker, who is technical sales and marketing advisor, and Hunter, who is technical service manager, explained that there is a wide variety of corrosion inhibitors that protect specific metals. Rust is the common name for the electrochemical oxidation of iron and its alloys; rust inhibitors retard this kind of corrosion of ferrous metals. Baker singled out rust because lubes, greases and metalworking fluids frequently are in contact with steel, and the corrosion of steel is particularly destructive. Other corrosion inhibitors protect nonferrous metals, such as yellow metals (copper and its alloys) and aluminum alloys. Many fluid lubricant formulations contain approximately 0.1 percent or lower levels of corrosion inhibitors, while greases may contain 0.5 to 3 percent.
Hunter and Baker noted that lubricant, grease and metalworking fluid formulations rarely contain fewer than three performance-enhancing additives, and usually more. Many rust and corrosion inhibitors are surface active molecules; they adsorb to the metal surface – adhere in a thin layer – and protect it from attack by aggressive chemicals, water and oxygen that cause corrosion. However, antiwear and extreme pressure additives also must adsorb to reduce friction, and they compete for real estate on the surface. Formulators must find combinations of additives that are compatible or have positive (synergistic) interactions and avoid those with negative (antagonistic) interactions.
Figure 1 lists common positive and negative additive interactions. For example, certain chemistries effectively inhibit rust but have a negative influence on water sensitivity, which adversely affects lubricant demulsibility, hydrolytic stability or filterability in the presence of water. Some formulations contain multiple inhibitor chemistries to improve performance and/or cost-effectiveness. For example, sulfonates and carboxylates frequently function synergistically for rust protection.
There is a large toolbox of corrosion inhibitors on the market, Gaston A. Aguilar, Ph.D., senior research and development scientist at Richmond, Virginia-based Afton Chemical, told LubesnGreases. For gear oils and greases, corrosion inhibitors are in general quite effective. However, formulators may increase treat rates as the environment becomes more severe, and corrosion inhibitors may affect how other additives work by competing for metal surfaces, interfering with antiwear and extreme pressure additives, and affecting demulsibility and foam.
Aguilar emphasized that it is particularly important to select corrosion inhibitors that are compatible with the thickener system of a lubricating grease. Corrosion inhibitors can affect the structural stability of grease thickeners and grease consistency. Certain inhibitors provide effective corrosion performance but shorten the useful life of grease in standardized test rigs, such as the FE9 angular contact ball-bearing tester. This could be due to multiple factors, including the effect of corrosion inhibitors on the greases mechanical and oxidative stability, and corrosion inhibitor interference with antiwear/EP additives.
Paul A. Bessette, president of Triboscience & Engineering in Fall River, Massachusetts, explained that corrosion inhibitors are essential for gear oils because steel gears are prone to corrosion. For example, in one experiment, 52100-alloy steel balls underwent severe corrosion while immersed in deionized water for several months at ambient temperature. In contrast, steel balls coated in gear oil formulated with 0.1 percent ashless corrosion inhibitor resisted corrosion in the same test.
Bessette agreed with Baker and Aguilar that formulators need to experiment to find appropriate corrosion inhibitors for specific lube chemistries. For example, greases thickened with clay particles present a special challenge. Many corrosion inhibitors adsorb onto clay particles, and formulators may add an extra amount to compensate.
However, this can create a rheological time bomb, said Bessette, if additives interfere with the network of clay particles that provide grease structure. Then, clay grease consistency can gradually decrease from NLGI grade 2 to grade 1 or lower during storage over several months.
Other additive interactions only appear to be unfavorable. Bessette developed a corrosion inhibitor that caused discoloration in gear oils formulated with certain sulfur-containing extreme pressure additives. This color change had no effect on performance, although it resembled the kind of discoloration that often signals oxidation and chemical breakdown.
Bessette also pointed to the importance of a particular class of corrosion inhibitors for nonferrous metals. Metal passivators, such as benzotriazoles, protect so-called yellow or copper-based alloys. Otherwise, corrosion would release copper ions that can catalyze oxidation of base oils and shorten the lubricants useful service life.
While many effective corrosion inhibitors are available on the market, Bessette noted that there are some gaps. The market would welcome more economical corrosion inhibitors for PFPE (perfluoropolyether) greases. PFPE greases are used at very high temperatures that degrade conventional corrosion inhibitors.
Other challenges loom on the horizon. Baker commented that if the U.S. Environmental Protection Agency eliminates medium- and long-chain chlorinated paraffins from metalworking fluids, as it has proposed to do, replacing those extreme pressure additives with sulfurized chemistries may require greater attention to the corrosion of yellow metals by active sulfur.
The supply of naphthenates has become problematic, too, according to Baker. Metal salts – primarily the zinc salt – of naphthenic acid, a refinery by-product, are effective and inexpensive rust inhibitors for some applications (mainly greases). Less naphthenic acid suitable for lubricant use is being produced, and while alternatives are under evaluation, they may increase cost.
Aftons Aguilar noted that certain additives, such as barium sulfonates, are falling out of favor because of concerns about regulations on barium, and are no longer in use in certain parts of the world. For metalworking fluids, boric acid and its derivatives, as well as certain amines, have also run into regulatory issues. They havent gone away, but people are looking for replacements, he commented.
The Feb. 24 issue of Lube Report revealed another additive at risk. Invista announced plans to close its Victoria, Texas, plant on March 31. The plant manufactures C12 intermediates, which include Corfree M1, a corrosion inhibitor that is widely used in metalworking fluids. According to Baker, There are numerous alternatives, and [we] can speculate that if the largest volume additive becomes unavailable, the cost of treatment is very likely to be less economical.
Hunter, Baker, Aguilar and Bessette agreed that regulatory barriers are discouraging additive suppliers from developing new corrosion inhibitors for the lubricant industry. Baker elaborated: The effort and cost of registration of any new compounds inhibits development of new individual additives, and to some extent significant new uses of existing compounds. The current challenge continues to be locating combinations of existing chemistries to meet increasingly severe requirements. Also, required labelling for hazards has caused some end users to discontinue use of some existing chemistries that perform well and are not prohibited.
Aguilar added that regulatory and testing requirements differ in the U.S., the European Union, China and Japan. As a result, companies are not developing new corrosion inhibitor chemistries or designing new molecules. Instead, the lubricant industry is hoping to discover new synergies by formulating combinations of existing additives.
Bessette concluded, We are always willing to test new corrosion inhibitors and see their advantages. We are always willing to look under the tent.
Mary Moon, Ph.D., is a physical chemist with hands-on R&D, management and problem-solving experience in the lubricating oil and grease and specialty chemicals industries. Contact her at firstname.lastname@example.org or (267) 567-7234.