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Additives can beat the creeping red peril

10th May 1980, Page 44
10th May 1980
Page 44
Page 45
Page 46
Page 44, 10th May 1980 — Additives can beat the creeping red peril
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Which of the following most accurately describes the problem?

Dr C A Smith looks at the action of additives on lubricants,

THERE are many hundreds of different varieties of lubricants, many of them tailored to meet particular requirements. Lubri cating greases are solid or semi-solid lubricants made by thickening agents. Synthetic lubricants, which will operate over a very wide range of temperature, have been developed mainly for aviation gas-turbine engines. These are generally carboxylic esters and are very expensive products.

The main function of most acid contamination in lubrilubricants is to reduce friction cants.

and wear between moving surfaces and to abstract heat. They also have to remove debris from the contact area. This includes combustion products in an engine cylinder, swarf in metal-cutting operations. Sometimes they have to protect the lubricated or adjacent parts against corrosion, but this is not a prime function of most lubricants.

On the other hand, many lubricants do contain corrosion inhibitors and some lubricating oils, greases, mineral fluids and compounds are specially formulated to prevent the corrosion of machinery or machine parts, particularly when these components are in storage or transit. These are temporary protectives.

Oxidation

Lubricating oils deteriorate in service in two ways, they become contaminated and they undergo physical and chemical changes due to oxidation. In engines the common contaminants are airborne dust and wear products, unburnt fuel, fuel combustion products and water.

The oxidation products are mainly acidic materials and asphaltenes. Asphaltenes in association with fuel contaminants and water form sludges and lacquers. The acidic materials resulting from oxidation are generally weak organic acids although in extreme cases strong mineral acids may be produced. However, contamination by fuel combustion products is the source of almost .all strong Thus, distillate diesel fuels can contain up to 1 per cent sulphur (by weight) and residual diesel fuels up to 3 per cent sulphur. This sulphur is oxidised to sulphur acids, and sulphuric acid condensate may be encountered on the cooler surfaces.

In gasolines the sulphur content is very low but halogen compounds are added to the gasolines as scavengers for the lead-based anti-knock compounds. These halogen compounds can give rise to halogen acids.

Lubricants with a high aromatic content tent to oxidise to give sludge-forming compounds, although some naphthenic oils give organic acids. Parafinic oils oxidise more slowly to give weak acids. In a plentiful supply of oxygen, oxidation proceeds at a significant rate at temperatures above about 60-130°C depending on the composition of the lubricant.

This oxidation is an extremely complicated process which involves the formation of organic peroxides as intermediates. It is catalysed by the presence of metals, particularly copper, iron and lead.

New methods

The progressive development of engines resulting in more arduous operating conditions, and particularly the use of longer oil-change periods, means that neither straight mineral oils nor compounded oils (mineral oils to which a proportion of an animal or vegetable oil has been added) are adequate for modern service requirements. Despite the introduction of new, improved methods of refining it has been necessary to enhance the performance of lubricants by the use of additives, either to reinforce existing qualities or to confer additional properties. Once additives were regarded with some suspicion — an oil that needed an additive was necessarily an inferior oil; today they are an accepted feature of lubricants.

Anti-wear

Almost all quality lubricants on sale today contain one or more additives. An enormous range of additives are available for use in lubricants, some produced by the oil companies and other provided by specialist manufacturers.

Additives are usually named after their particular function', but many additives are multifunctional. Thus, an anti-wear additive may also protect a surface against corrosion. The main types, of additives that can enhance the anticorrosion behaviour of lubricants are listed in Table 1.

Many additives, essential to the performance of the lubricant, provide no corrosion protection and some additives may become corrosive in certain circumstances. Thus EP (Extreme Pressure) additives contain chemical groups which are designed to react chemically with metal surfaces when normal lubrication fails, forming easily sheared layers of metal oxides, sulphides, chlorides or phosphates, thereby preventing catastrophic wear and seizure.

Reaction between EP compounds and metal surfaces should only occur at local hot spots and the layers formed are extremely thin. However, if the oprating conditions are very severe these layers are continually generated and removed as they fulfil then anti-wear function.

A process of this nature is sometimes called 'chemical wear, and if sliding surfaces operate continually undet these conditions loss of metal from the rubbing surfaces can ultimately result in failure, Alternatively all the EP additive may be used up (depleted) and then failure by seizure will occur. EP agents are intended to cater for the occasional over-load condition and it must be emphasised that machinery should be designed so that it does not require the continual action of EP agents to junction satisfactorily.

Obviously the selection of an EP additive requires great 'care; if it is too active it may give rise to excessive metal removal under normal operating conditions (see the section on corrosion by sulphur additives). Also, if a component is prone to fatigue pitting in service the presence of an over-active EP agent may result in corrosion fatigue.

Modern high-performance lubricants contain a number of additives, each with a particular, special function. Thus a high-grade diesel-engine lubricant may contain a VI improver, a dispersant, an anti-oxidant, a corrosion inhibitor, a basic compound, a pour-point depressant and an anti-form compound.

Inhibitor

Sometimes these additives may have undesirable side effects or interact adversely; in an oil the rust inhibitor may act as an emulsifier interfering with demuisification; in a diesel lubricant the dispersant may promote oil oxidation_ Frequently anti-corrosion additives may not be able to exert their maximum effect because they are competing for sites on metal surfaces. The development of a successful new lubricating oil requires much skill and experience and always necessitates considerable laboratory and field testing in order to strike the right balance between the various additives.

Sulphur

Oil additives may be incorporated in the oil phase in a grease. However, with greases there is also the opportunity to employ oil-insoluble anticorrosion agents, since these can be incorporated in the grease along with other thickening agents, e.g. sodium nitrite is incorporated in greases in this way.

Additives are consumed or 'depleted' as they fulfil their respective functions. It is for this reason, more than any other, that it is necessary to change engine oils regularly.

Sulphur compounds occur naturally in most lubricants and many oil additives contain sulphur. In a properly formulated lubricant these sulphur compounds should be inactive at ambient temperature.

At elevated temperatures they may decompose to give more active materials which can stain and corrode metals, particularly silver and copper. However, these same sulphur compounds have many beneficial qualities; this is why they are not removed completely in refining and why they are used as additives.

Thus sulphur compounds in lubricants generally act as anti-oxidants, preventing acid and sludge formation. They also form very thin films on metal surfaces protecting them from acid or peroxide attack. In addition, sulphur compounds are often used as EP agents. The oil chemist must try to strike a balance — the activity of the sulphur must be high enough for it to exert a beneficial effect and yet not so high as to stimulate corrosion.

Silver

All too frequently lubricants containing sulphur are exposed to more severe operating conditions than intended, and staining and corrosion results. This has given sulphur compounds and sulphur-containing additives, particularly those of the dithiosphosphate type, a bad name.

Silver bearings are still used in certain diesel engines and if the lubricants for these engines contain sulphur CQMpounds with too much chemical activity, severe corrosion ensues. A more widespread problem is the corrosion of phosphor-bronze alloys (containing about 10 per cent tin) particularly in little-end bushes in diesel engines where temperatures can exceed 200°C.

Some engine builders and operators hold sulphur additives entirely responsible, but this opinion cannot be substantiated, for corrosion can occur with lubricants containing only natural sulphur compounds.

Copper

Two important metallurgical factors affecting the corrosion resistance of phosphorbronze alloys are the amount of alloying element in solution in the copper-rich phase and the porosity of the alloy. For example, if the amount of tin in solution can be increased by special casting techniques or heat treatments the corrosion resistance is greatly increased. Similarly, zinc or silicon in solution also increases the resistance of copper to sulphur corrosion. If the alloy is porous the lubricant is drawn into the pores where it stagnates, and, at high temperatures, becomes very corrosive, e.g. copper catalyses oil oxidation with the consequent formation of corrosive sulphur compounds.

The most satisfactory solution to this problem is to employ a Corrosion-resistant alloy, and alloys of the gunmetal type containing 2-4 per cent zinc, have proved corn pletely satisfactory. The substitution of zinc for phosphorus gives sounder castings and improves the corrosion resistance of the copper-rich matrix.

Engine lubricants are exposed to severe operating conditions, being subjected to high temperatures, the products of combustion and a plentiful supply of oxygen. Consequently, unless the oil is changed at appropriate intervals strong mineral acids and weak organic acids may accumulate. In addition, droplets of water may be formed and these can contain strong mineral acids derived from the fuel combustion gases. These most affected by this condensed moisture.

A special dynamic corrosion test has been developed to study corrosion in these twophase (water-in-oil) systems. Problems associated with the droplets sometimes give rise to emulsions which deposits in the colder portions of an engine, e.g. on the rocker-box covers; ferrous surfaces are retention of water in engine lubricants are likely to become, more acute as anti-pollution devices are fitted to engines.

Lead

The harmful effects are best countered by anti-rust and basic additives, and in diesel engines burning high-sulphur fuels, e.g. marine diesel engines, very high levels of basicity are required.

Cast or sintered copper-lead or lead-bronze alloys are widely used for engine bearings. The lead phase in such bearings is readily attacked by weak organic acids and almost all the lead can be leached out unless preventative measures are employed. The lead may be protected by a precision electro-deposited overlay of a lead-tin or lead-indium alloy.

About 3 per cent tin or 5 per cent indium in lead will render the lead resistant at attack by oil-oxidation acids. One reason why leaded bearings are protected by an overlay, and not by incorporating the protective alloying elements in the underlying lead, is that both tin and indium dissolve preferentially in copper. In a cast or sintered bearing, therefore, any tin or indium will be found in solution in the copper-rich phase, leaving the lead-rich phase susceptible to attack.

Moisture

In overlay bearings operating above about I40°C, the tin or indium in the overlay diffuses towards, and alloys with, the underlying copper, depleting the overlay and reducing its resistance to corrosion. This depletion by diffusion can be combatted by the use of a diffusion barrier or 'dam', e.g. a nickel-rich layer between the bearing alloy and the overlay.

In gasoline engines lead halides accumulate in the lubricant; occasionally these give rise to the corrosion of aluminium-alloy pistons and very rarely to corrosion on aluminium-tin bearings.

In addition to the usual oxidation and corrosion inhibitors, lubricants for heavily loaded gears almost always contain EP additives containing sulphur, chlorine or phosphorus. In order to function, these additives must react locally with the metal surfaces, an yet the extent of the reaction should not be such that it could be described, as corrosive, or promote fatigue pitting. These EP additives may be quite safe with ferrous metal surfaces, but may cause severe corrosion on copper alloys, e.g. on bronze worm wheels, if for any reason excessive temperatures arise.

Some turbine lubricants have to lubricate the turbine gears as well as the turbine; in these circumstances any EP additives employed should not be corrosive in the presence of moisture.

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