Lubricant corrosion inhibitors: materials and mechanisms of action

Among the many functions of any lubricant, corrosion inhibition is one of the most widely required. However, the wide variety of applications requiring corrosion protection also entails a wide variety of chemistries.

Situations which require corrosion protection include:

Long-term storage or transport of equipment or parts
All forms of metalworking, sometimes with corrosive fluids
Regular hot/cold operating/idle cycles that give rise to condensation in equipment

Sometimes transport or use is in corrosive environments, such as at sea or port-side; sometimes the equipment is exposed to acidic combustion gases. To further complicate matters, the chemistry that protects one metal in one circumstance is often ineffective in protecting another metal in the same circumstance or the same metal in a different circumstance.

Finally, as many corrosion preventives are surface-active, the formulator must consider that a superb corrosion preventive for one application is near useless for another, because it interacts too strongly with the surfaces in the contact or the other ingredients of the lubricant. Thus, it can compromise another function, such as friction reduction (metal surface) or foaming (oil/air interface). Possibly even worse, it can complex with other additives or contaminants to create new species that cause harm, such as by blocking filters.

On the positive side, there are circumstances when a formulator may not require a separate corrosion inhibitor, as other ingredients of the formulation have corrosion prevention as a secondary function. Examples include sulfonates and fatty amides, which are added primarily as detergents and friction modifiers respectively.

Almost all corrosion inhibitors now used in lubricants are organic compounds or salts of organic compounds. Inorganic salts, such as chromates or arsenates, are no longer used in lubricants, but are common in metal finishing.

Mechanisms of action

To appreciate how corrosion preventives work, it is important to know that corrosion of a metal requires the presence of an oxidising agent and an electrolyte. Oxidising agents can be acids, bases, oxygen or sulfur. The electrolyte is most commonly water. If the oxidising agent or electrolyte can be prevented from contacting the metal, then corrosion doesn’t occur.

1) Passivation

The most common mode of action of corrosion inhibitors is to block the surface of the metal part from interaction with water or oxygen. Examples of pure physical blocks include oxidates, oxidised waxes and triazoles. The process of blocking access to the surface is called passivation, although the term is most often heard when referring to copper or yellow metal passivators.

Examples of oxidates include Sonneborn’s OXPET. Chemceed and NCeed are suppliers of various benzo- and tolytriazoles.

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2) Neutralisation

Corrosion inhibitors may also neutralise corrosive materials in solution.

Nitrogen-containing additives, such as alkanolamines, amines or imidazolines have strong affinity for metal or metal oxide surfaces, but are also basic, so can neutralise acids in solution.
Carboxylic acids can be very good corrosion inhibitors in some circumstances by binding to surfaces and neutralising alkaline species in solution.

Angus Chemical Company sells alkanolamines in its CORRGUARD^® range. The MAXHIB range from PCC Chemax Inc and some of the PEL-HIB™ range from Ele Corporation contain amine carboxylate salts and alkanolamides.

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Perhaps counter-intuitively, corrosion inhibition may also be achieved by oxidising agents themselves. Phosphating steel, for example, raises the oxidation state of iron to Ferric (Fe³⁺), salts of which are less soluble than the corresponding Ferrous (Fe²⁺) salts.

3) Corrosion prevention as a secondary effect

Neutralisation in an automotive crankcase is usually described as a primary function of an additive with little reference to the resultant benefit of reduced corrosion. Those selling anti-oxidants also focus on the primary effect, omitting that anti-oxidants prevent corrosion by reducing oxidising agents in solution.

Emulsifiers prevent water reaching the metal surface, so ethoxylated amines or ethoxylated amides are often marketed as emulsifiers that have some corrosion preventive properties. Fatty acid amides, primarily thought of as friction modifiers, passivate surfaces and consequently prevent corrosion.

Ele Corporation’s PEL-AMID™ range and KLK Oleo’s Hedilub-EMEA/045 are examples of emulsifiers of different chemistries that also provide corrosion protection.

Search all emulsifiers with corrosion protection in Prospector.

Phosphates are added to many crankcase and industrial lubricants formulations as extreme pressure or anti-wear additives, but also provide effective corrosion prevention. Neutral phosphates (usually triaryl phosphates, such as tricresyl phosphate or trixylyl phosphate) decompose in solution and precipitate onto the surface of the metal as a coherent film, which prevents oxidation of the metal. When an amine phosphate is used in formulation, then the amine can also be active as a corrosion preventive, neutralising acids in the fluid or adsorbing onto the surface.

Sulfonates

Possibly the most common rust preventive chemistry is the various forms of sulfonate. And there is significant variety: natural or synthetic sulfonates, neutral or overbased, benzene or naphthalene, and delivered as sodium, magnesium, calcium or barium salts.

Even with the same base number and metal, the source of the sulfonate itself can cause variation in activity. Synthetic sulfonates from different manufacturers could have different chain lengths, aromatic group (benzene or naphthalene), alkyl chain substitution, branching or chain length distributions. Natural sulfonates could have been produced as by-products of the petroleum refining process, or white oil production.

It is probably no surprise that one manufacturer’s material might not act in the same manner as a competitor’s material with the same headline specification. Consequently, learning about the sulfonate that works for you and some of its near-neighbours is very important if you anticipate a need to switch to an alternative.

The main attraction of sulfonates is their multiple modes of action. Sulfonates can be overbased to deliver a large alkalinity reserve in the form of metal carbonates, which react with acids in solution. Additionally, the sulfonate and the counter-ion can both bind to the metal (oxide) surface.

The effectiveness of a sulfonate as a rust preventive can depend on the counterion. Of the commonly available sulfonates, barium sulfonates are often more effective than calcium salts, which are often more effective than magnesium or sodium sulfonates. However, as water-soluble barium compounds are poisonous, their use is undesirable in some countries, though not yet banned.

Sonneborn markets natural sodium sulfonates under its PETRONATE brand. Pilot Chemical Company’s Aristonate^® range are synthetic sodium sulfonates.

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Corrosion in transport

A common means of coating surfaces for transport is to use vapour phase corrosion inhibitors (VPCIs, or volatile corrosion inhibitors, VCIs). These volatile and polar additives are easily introduced into closed equipment, pipes, tanks or equipment in sealed bags and find their way to all metal surfaces, coating them with a monolayer that prevents water and corrosive agents from contacting the surface. If the VPCIs are impregnated into paper, then opening and closing the bag does not necessarily initiate corrosion, as the VPCI is replenished by further evaporation from the paper.

Volatile nitrogen-containing materials, such as imidazolines, imidazoles or triazoles are very effective as VPCIs and as additives in lubricants.

Metalworking

Several metalworking processes are carried out with water-based emulsions (soluble oils). Therefore, corrosion protectives are required where water is intentionally present and there is a high degree of local heating due to the metalworking process. The preferred chemistry to prevent corrosion includes amine carboxylates, fatty amides and fatty alkanolamides.

An additional issue with water-based fluids is the interaction of the additives with the naturally-occurring salts in the water. This can lead to staining, so careful screening of water samples is often important. This screening may have to take place several times during the year, as the mineral content of the local water supply will vary with the seasons (rainfall, snow melt etc.)

Where cutting involves chlorinated paraffins, the presence of any humidity is likely to result in generation of hydrochloric acid, so immediate neutralisation is required, often by using a sulfonate.

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