Corrosion Inhibitors Chemistry

Nitrite:

Sodium nitrite (NaNO₂) was used as an anodic inhibitor to prevent internal corrosion by water and air in steel pipelines for petroleum products. It was first speculated that the mechanism involved the formation of a tight oxide layer of oxygen and nitrite to prevent corrosion by forming a passivating layer, but subsequently it was proposed that the nitrite accelerated the production of Fe³⁺ on the metal surface and formed a less-soluble protective barrier than the F^e2+ species.

Historically, nitrites were primarily used in aqueous systems such as water treatment or concrete emulsions as an anodic inhibitors, but subsequently they have also been used in soluble oils for metalworking applications. Since the 1950s, the reaction of amines commonly used in the soluble oils with nitrite was found to form the carcinogenic nitrosamines (R₂NNO) under acidic conditions.

2HNO2 → N2O3 + H2O

R2NH + N2O3 → R2N−N=O + HNO2

As a result, the use of nitrites as RPs in metalworking fluids containing ethanol amine carboxylate salts was banned by the U.S. Environmental Protection Agency (EPA) in 1984, and the industry quickly modified their formulations to replace the nitrite with other inhibitors. Typical replacements used were the borate, carboxylate, or phosphate salts of triethanolamine (TEA). Subsequently, it was found that of the three common amine ethoxylates (monoethanolamine [MEA], diethanolamine [DEA], and TEA), DEA was the most prone to the reaction with nitrite, whereas there was no evidence of nitrosamine formation in the TEA-containing soluble oils. Despite low nitrosamine formation with TEA, the use of nitrite has been widely discontinued in metalworking applications.

Chromates:

Chromates are another inhibitor that is part of the class of anodic passivators, which appear to inhibit corrosion through the formation of an oxide coating. Historically, chromates have been used in steel plating and finishing, aqueous corrosion inhibition, and leather tanning, and although there have been attempts to make oil-soluble chromate derivatives, they have not been widely used in lubricants due to their instability in the presence of organics. In water treatment, these products were applied as chromic acid, sodium chromate (Na₂CrO₄), or sodium dichromate (Na₂Cr₂O₇), all of which contain hexavalent chromium. Subsequently, the hexavalent chromium (Cr⁶⁺) was found to be a powerful respiratory carcinogen and possessed high aquatic toxicity. As a result, the use of chromates in aqueous water treatment was banned in 1990, and interest in using them in lubricants has disappeared.

Hydrazines:

Hydrazine (N₂H₄) has been historically used for water treatment as a corrosion inhibitor and oxygen scavenger. The hydrazine prevents corrosion in a boiler by (1) reacting with O to form N₂ and H₂O; (2) reacting with Fe₂O₃ (hematite) to form the harder (passive) Fe₃O₄ (magnetite), which makes a protective skin over the iron surface; and (3) forming NH₃ at high temperatures and pressures to maintain a high alkalinity. Although it has been demonstrated that derivatives of hydrazine can be used to inhibit copper corrosion in sulfuric acid, due to health concerns there relatively few new oil-soluble derivatives have been developed, and the main focus has been to use alternative organic treatment chemicals.

Silicates:

These materials are mainly used as inhibitors in water treatment for potable water due to their low cost. Owing to their poor oil solubility, silicates have limited use as inhibitors in lubricant formulations.

Oxidates:

Oxidates are one of the oldest classes of RP additives and were historically made from oxidized oils, waxes, and petrolatums obtained from the refining process. The production of oxidates can be accomplished by either heating the petroleum product air in a closed vessel in the presence of a catalyst or chemical treatment using nitric acid, sulfuric acid, or KMnO₄. Both these methods typically make a combination of polar compounds (including carboxylic acids, esters, alcohols, aldehydes, and ketones), where the total acid number (TAN) of the resulting mixture is between 10 and 200 mgKOH/g and the saponification number (SAP#) is between 10 and 200 mgKOH/g. Typically, commercial grade products possess a TAN of 50 and 100 mgKOH/g and a SAP# of 10 and 50 mgKoh/g. Depending on the application, oxidates can be applied to a surface in various methods as follows:

Aqueous dispersions: The oxidate can be dissolved or suspended in a water-based formulation that is then applied to the surface. Either the water is removed with washing or the film is allowed to dry to make an RP coating.

Solvent based: The oxidate is dissolved in a low-boiling solvent (like naphtha), and the solvent is allowed to quickly evaporate to form the RP coating.

Solid film (typically wax): Either the oxidate is heated to apply the coating as a liquid and then allowed to cool to solidify or the solid oxidate is applied through mechanical methods as a solid at ambient temperatures.

Although these materials can be used as effective RP additives in their acid form, they are typically neutralized to form more effective coatings. It has been found that oxidized wax neutralized with Ca(OH)₂ or Ba(OH)₂ are effective RPs, and that the use of Ca(OH)₂ as a neutralizing agent is effective at preventing gelation. Basic materials that contain CaCO₂ have also been found to be effective when combined with petroleum oxidate. The combination of petroleum oxidates and over-based calcium sulfonates has demonstrated both improved lubricity and corrosion protection in forming and engine oils. Additionally, the petroleum oxidates can be neutralized with amines to form the carboxylate amine salts. Although there are various amines listed in the patent literature (piperadines and polyamines), the most common amines used are the simple alkanolamines.

Sulfonates:

The sulfonates comprise a class of compounds that can be derived from petroleum (natural) or synthetic feedstocks. The sulfonic acids are formed in the reaction of SO₃ with a feedstock and can be neutralized with various bases to form the Na, Ca, Mg, or Ba salts, all of which have demonstrated activity as RPs in various applications. Additionally, the neutral salts can be over-based by the addition of excess base and carbon dioxide. For example, in the case of a calcium petroleum sulfonate, an excess of Ca(OH)₂ and CO₃ can be added to the sulfonic acid to form a colloidal suspension of CaCO₃ in oil, where calcium sulfonate serves to disperse the CaCO₃ in the oil carrier.

The over-based sulfonates serve a dual role in rust prevention, where the sulfonate acts to form a protective layer, and the calcium carbonate acts to absorb any acidic by-products of corrosion. As a result, a combination of neutral and over-based sulfonates can be a quite effective RP. The two types of sulfonates that are commonly manufactured are as follows:

A natural or synthetic sulfonate can be neutralized and over-based with various cations depending on the application. For example,

Sodium sulfonate: In general, the high equivalent weight (500–550 EW) petroleum or (390–700 EW) synthetic alkylbenzene sulfonic acids are neutralized with NaOH to form sodium sulfonates, which are commonly used in soluble oils for metalworking applications, where the divalent cations (Mg, Ca, and Ba) are detrimental to the stability of the soluble oil. The sodium salts of the synthetic dialkyl naphthalene sulfonates have been used, but their high cost has limited their use in this application.

Calcium sulfonate: Both the synthetic and natural sulfonic acids have been neutralized with CaO or Ca(OH)₂ to form the neutral calcium sulfonates, but generally these products are more effective when they have been over-based. The over-based calcium sulfonates can contain either amorphous or crystalline form of calcium carbonate. The amorphous calcium carbonates are easily soluble in oil and are commonly used as a detergent in engine oils, whereas the crystalline form of calcium carbonate, called calcite, typically contains colloidal particles that are too large to be held in suspension in the fluid and precipitate to form a gel or gelled solid and are commonly used in RP coatings and greases.

Magnesium sulfonate: Both the natural and synthetic sulfonic acids can be neutralized with MgO or Mg(OH)₂ to form the magnesium sulfonates. Although magnesium sulfonates have not been generally used in RP oils, the over-based magnesium sulfonates are extensively used as fuel oil additives. In particular, the undesirable contaminants in fuel oil such as V and Na can form low-melting corrosive slags on the fi reside of a commercial boiler used for power generation. The molten V2O5 can act as an oxygen carrier and can accelerate corrosion. The addition of the over-based magnesium sulfonate to the fuel oil serves to react with low-melting sodium vanadates to form high-melting magnesium vanadates that increase the viscosity, reduce the oxygen uptake, and counteract the destruction of the protective oxide film. Additionally, magnesium sulfonate reacts with sulfur oxides (SO₃/SO₂) forming high-melting friable MgSO₄, which can be easily removed by washing, whereas the carbonate neutralizes any free acids to reduce the pH and lower the acid deposition rate (ADR).

Barium sulfonate: Both the natural and synthetic sulfonic acids can be neutralized with BaO or Ba(OH)₂ to form the barium sulfonates. The barium sulfonates have been found to be effective when both neutral and over-based barium sulfonates are combined in a formulation and applied to a metal surface, where these products are generally used as RPs in mill and slushing oils. The BaDNNS was found to be very effective at low concentrations compared to Ca and Na derivatives, and, in general, the RP effectiveness of the sulfonate increases with ionic radius where Ba > Ca > Mg > Na.

Type	Solubility	Ionic	ΔHsoln	ΔHlattice
	Product (Ksp)	Radius (pm)	(kJ/mol)	(kJ/mol)
MgCO3	6.82 × 10-6	65	-25.3	3180
CaCO3	4.96 × 10-6	99	-12.3	2804
SrCO3	1.1 × 10-6	113	-3.4	2720
BaCO3	2.85 × 10-6	135	4.2	2615

The effectiveness of alkaline earth metals can be correlated to their ionic radius, which is inversely proportional to the solubility of the metal carbonate (and metal sulfonate) of the additive. For example, the large ionic radius of Ba results in a small release of energy (enthalpy of solution) due to the small amount of solvation necessary for this large polarizable cation. As a result, the large cations require more energy to solubilize and are more difficult to remove from a metal surface by a humid atmosphere or aqueous washing.

Carboxylates:

Carboxylates are some of the oldest known corrosion inhibitors and can be made from animal (lanolin, lard, or tallow), vegetable (tall oil fatty acids [TOFAs]), or mineral (naphthenic or aromatic acids) oils and were commonly used in early slushing oil formulations. The carboxylic acids can be neutralized with many exotic cations (such as Bi [54]), but they are commonly reacted with NaOH to form the sodium carboxylate salts.

Owing to their corrosivity in aqueous systems, carboxylates are more typically combined with alkanolamines to form the alkanolamine salts in situ in soluble oil applications for metalworking. In metalworking, the use of the alkanolamine carboxylates provides good coupling with other lubricity additives, enhanced lubricity, and the formation of soft (noncalcium containing) residues, but the alkanolamine carboxylates do suffer from hard water sensitivity, and many shortchained derivatives can cause odor and excessive foaming. The dicarboxylates, which possess good corrosion protection and low foam, can remedy this problem, but they are not popular due to their high cost and poor coupling with other lubricity additives.

In other applications, carboxylates have been used with varying amounts of success, and it had been found that a small amount of the C₆–C₁₈ carboxylic acids prevented corrosion in turbines, whereas acids with chain length <C₆ promoted corrosion. Additionally, in stamping applications, it was found that a simple fatty acid ester provides good lubricity as well as possesses inherent RP properties.

Alkyl Amines:

The alkyl amines represent the largest and most diverse chemistry of all the RP types and are used in various metalworking and industrial oil applications. The most common alkylated amines that are used include MEA, DEA, and TEA, fatty amines, diamines, phenylene diamine, cyclohexylamine, morpholine, and ethylenediamine, triethylene tetramine (TETA), and tetraethylene pentamine (TEPA). It had initially been recognized that the amine was preferentially adsorbed onto the metal surface and inhibited the reaction of the corrosive species and the metal, but now it is believed that the alkyl amines first displace the water on the metal surface to form a bond between the lone pair on the nitrogen and the unoccupied orbitals on the metal through defects present in the oxide coatings. As a result, the amines provide cathodic protection by creating a barrier and inhibiting H₂ formation in acidic environments. In general, it has been found that longer-chained amines are more effective than the shorter-chained amines and that the nucleophilicity of the nitrogen strongly correlates with rust inhibition effectiveness where the tertiary is more effective than the
secondary or primary amine.

Owing to their high volatility and low ash formation, products such as cyclohexyl amine, morpholine, and piperadine can be used as VCIs. In systems that are unsuitable for oil, grease, or hard coatings, they expand in the vapor phase to fill the void space in a metal enclosure to form an extremely thin fi lm over the entire metal surface (including intricate interior parts in a fired engine).

Owing to the absence of inorganic salts (such as the phosphates, borates, or carboxylates), they do not have the tendency to leave dry residues on the surface [64]. They are also commonly found in sweet (CO₂) and sour (H₂S) gas applications where they volatilize to fill the entire void space of the pipeline.

Unfortunately, it has also been found that the amines tend to cause occupational skin diseases (including irritant and allergic contact dermatitis) in workers exposed to metalworking fluids. In particular, the commonly used mono- and diethanol amines were found to elicit a significant amount of positive reactions, and as a result, they have been largely phased out of this application in favor of the tertiary amines such as TEA. The tertiary amines possess the added benefit of low ecotoxicity and are used environmentally sensitive applications. For example, they are particularly effective in oil fi eld acidizing operations as corrosion inhibitors where they are directly introduced to the outdoor environment.

Alkyl Amine Salts:

Although the alkylamines are commonly used in the gas phase, the amine salts of the carboxylates, borates, and phosphates are most commonly used in metalworking fluids where they are formed in situ by reaction with the corresponding acid. For example, the combination TOFA neutralized with triethanol amine to form the amine carboxylate salt is commonly used as a RP. Each amine salt formed for metalworking and RP applications possesses its own unique chemistry and performance issues, each of which will be discussed.

Amine Carboxylate Salts:

In metalworking formulations, typically tertiary amines such as TEA are neutralized with organic fatty acids to form the amine carboxylates in situ to not only provide corrosion inhibition but also improve lubricity and emulsification. Although there have been many combinations of carboxylic acid and amines proposed, the most effective combinations include the long-chained carboxylic acids from C₁₈ to C₂₂.

Amine Carboxylate Salts:

The borates represent the most chemically diverse and least understood of the RP classes. Although the structure of the borates and borate esters can be written empirically as B(OR)₃, where R is a H or alkyl group, the actual three-dimensional structure of the borates are complex chains and rings that contain both sp² (3-coordinate)- and sp³ (4-coordinate)-hybridized boron species. For example, the species Na₂(B₄O₅(OH)₄) contains both sp³ and sp²-hybridized species, where two of the sp²-hybridized B have empty p-orbitals that can be used to bond to an amine or any other lone pair of electrons as well as use the terminal oxygen to form borate ester linkages.

As a result, common ethoxylated amines, such as the triethanol amine, can bond to the borate using four different modes of connectivity (Figure 17.9), which can be described as follows:

The TEA can datively bond to the unoccupied sp²-hybridized orbital of the boron to make the borate salt that is nominatively of the form R₃N:B(OR)₃.

The TEA can bond through one oxygen on an sp²-hybridized boron to form the monodentate −(N−CH₂−CH₂−O−B)− bond.

The TEA can datively bond to the unoccupied sp²-hybridized orbital of the boron to make the borate salt and can bond through one oxygen on an sp²-hybridized boron to form the bidentate −(N−CH₂−CH₂−O−B)− bond.

The TEA can bond through two oxygen on an sp²-hybridized boron to form the bidentate −(N−(CH₂−CH₂−O)₂−B)− bond.

The advantages of the borate salts are their low cost, low toxicity, hard water stability, excellent antiwear, and reserve alkalinity. The large disadvantage is the possible formation of a tacky residue that remains after the part is machined due to the partially dehydrated products such as meta-boric acid and its esters depositing on the surface.

The borates not only serve to inhibit corrosion but have also been found to be bacteriostatic (biostatic) agents, and a large synergistic inhibitory effect has been found when combined with amines. Although the mechanism of the effect is still unclear, it is believed that it may be due to the release of boric acid at low pH that may react to form cis-diols with the ribonucleotides to promote the antimicrobial activity.

Although the borated amines have been cited for use in engine oils, hydraulic fluids, and slushing oils, they are most commonly used in metalworking fluids for both their rust inhibition and biostatic properties

Phosphates:

The phosphating of metals is a well-known technique to improve both wear and corrosion resistance and is primarily used as a surface preparation before painting. The process of phosphate coating is the treatment of iron, steel, or a steel-based substrate with a solution of phosphoric acid or as K, Na, or Ca phosphate salts, in the presence of heavy metal accelerators (Zn or Mg) whereby the surface of the metal is converted to a mildly protective layer of insoluble crystalline phosphate. Phosphating is considered the heart of pretreatment operations in a steel mill and where the top surface of the metal is converted into a highly insoluble, corrosion-resistant coating . The mechanism of this cathodic inhibitor is believed to be the precipitation of a phosphate film on the surface of the steel that prevents the corrosive action of acids and water. Additionally, phosphates have been extensively used in water treatment where the phosphate combines with calcium to form calcium phosphate precipitates.

Although the inorganic phosphates (mono-, ortho-, or polyphosphates) are not oil soluble, the phosphate esters can be synthesized to provide oil-soluble derivatives for lubricants. In particular, the tertiary and secondary phosphate esters of the form (RO)₃P=O or (RO)2OHP=O, where R is typically an alkylaryl group have been used in slushing oils alone or in combination with other additives. The most common alkylaryl derivatives include the tricresyl phosphate (TCP) and trixylyl phosphate (TXP). Although the trialkyl phosphates (such as tributyl or trioctyl) could be used for this application, the instability of these species due to facile thermal, oxidative, and hydrolytic degradation makes them less desirable as RPs.

The use of phosphates in soluble oils is less extensive, but there are a few examples of trialkyl phosphates being used in drawing and ironing operations. Although the phosphates possess low toxicity and excellent EP/AW and RP properties, their use in soluble oils for metalworking additives has been limited due to their stimulation of microbial growth that can lead to contact dermatitis or worse after prolonged worker exposure.

Phosphates have also been used in combination with amines, and this provides not only additional EP/AW protection but also both anionic (phosphate) and cationic (ammonium) RPs in a single salt.

Another area where phosphates have been extensively used is in aviation turbines in which low ash properties of TCP are desirable. Although the TCP produced in the 1940s and 1950s did possess a considerable amount of triorthocresyl phosphate (TOCP), which increased concerns of neurotoxicity, the TCP now commercially available can have TOCP levels as low as parts per billion (ppb) and are not a significant concern.

Nitrogen Ring Structure:

Although tertiary amines have gained wide acceptance in various RP formulations for their ability to neutralize acids, the nitrogen heterocycles have also found use in many common RP applications. The most common type of nitrogen ring structures are imidazolines, benzotriazoles, thiazoles, triazoles, imidazoles, and the mercaptobenzothiazoles, as well as any of their common derivatives that include a basic nitrogen in their structure. Owing to the volatile nature, these materials can be used as VCIs and are particularly effective in metal equipment such as engine blocks made from cast iron or aluminum. Additionally, the absence of an inorganic salt minimizes the possibility of dry residue formation.

It has also been found that some imidazoline derivatives have low toxicity in the marine environment and are useful in offshore oil and gas production [64,100]. Although the mechanism is still unclear, it appears as if the highly basic nitrogen in the ring structure is converted to a less basic salt (by reaction with organic acids), which renders the molecule less toxic in the marine environment.

Although many amine heterocycles have been proposed, due to their relative low cost and high RP effectiveness, primarily imidazoline and its derivatives have found use in grease, soluble oils, rolling, cutting, drilling, turning, grinding, wire drawing, stamping, and sheathing with varying levels of success. Additionally, the imidazolines can be used alone or combined with a number of organic acids to make the ashless amine salt, which have been demonstrated to be very effective in RP.