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Resisting Corrosion

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Simple over based sulfonate greases have been known from the middle of the past century. These were mainly used as anticorrosion coatings due to their tackiness and capability to inhibit corrosion processes, but other performance properties – such as oxidation stability -were poor.In this respect, simple overbased-alkyl salicylate greases do not differ highly from sulfonate ones.

Complex overbased greases, however, must be regarded in another category. Complex versions compare advantageously with simple overbased greases because they contain calcium metaborate (CMB) and 12-hydroxystearate (CHOS), as well as overbased sulfonate or alkyl salicylate. Complex overbased versions of these greases are on the same performance level, and by some properties exceed such multifunctional products as complex lithium, aluminum and polyurea greases.

Alkyl salicylate grease has strong anti-corrosion properties, and is useful for wire rope dressing and other demanding environments where corrosion and extreme-pressure protection has to be extremely high. This grease is used also for mining equipment, and is finding its way into applications such as lubricating and protecting industrial and home gas taps.

So far as protective properties of sulfonate greases are concerned, they are sufficiently high that these greases can often operate without anti-corrosion additives. But it is also known that overbased alkyl salicylate additives for motor oils have considerably better anti-corrosion properties than overbased sulfonate additives. Against this background, it was interesting to research corrosion preventive properties of alkyl salicylate greases, and whether they may offer greater performance in this area versus sulfonate greases.

SEEING INTO THE GREASE

First, a look into the structure of these greases is useful. As earlier research work into complex overbased sulfonate and alkyl salicylate greases has shown, CMB and CHOS are responsible for changes in the composition of micellae of overbased greases. This can be seen in the elementary particles of the greases structural skeleton: Micellae of simple overbased calcium sulfonate or overbased calcium alkyl salicylate are formed from nano-sized nuclei of calcium carbonate, modified with surfactant molecules of the calcium sulfonate (CaSu2 )oralkyl salicylate (CaAs2 ).

By contrast, the structure of the complex overbased greases is more elaborate. Here, molecules of CMB penetrate into the carbonate nucleus, and result in disordering of the sulfonate gel structure and noticeable softening of alkyl salicylategreases. MoleculesofCHOS,unlike CMB molecules, are included into the composition of micella shell and impart additional aggregative resistance to athixotropic system. (Thixotropy is the key property that makes a grease able to soften and move when shear is applied, and then instantaneously regain its former consistency after shearing is stopped.)

In consequence, complex sulfonate and alkyl salicylate greases, as distinct from simple overbased greases, are composed from micellae whose nuclei contain colloidal calcium carbonate and metaborate, and an adsorptive shell -either calcium sulfonate or alkyl salicylate and 12-hydroxy stearate, as shown in Figure 1.

Composition and structure of these complex greases are such that they successfully operate in many friction units without antiwear and extreme pressure additives. The main deficiency of simple sulfonate greases is their comparatively low oxidation stability,which makes it necessary to add antioxidants in order to improve their performance properties. Complex overbased alkyl salicylate greases have twice the oxidation stability by comparison,and thus can be used without additives in a number of applications.

TESTING THE MATERIAL

In an effort to determine the protective properties of sulfonate and alkyl salicylate greases, the Ukrainian Scientific &Research Institute of the Oil Processing Industry(MASMA) decided to study four model systems, which were the initial, intermediate and finished stages in the process of complex overbased grease formation. Model systems were prepared of simple overbased calcium sulfonate and simple overbased calcium alkyl salicylate greases; of these same simple greases which also contained one component of a complex grease (CMB or CHOS); and of complex versions of each, containing both CMB and CHOS. Thus, eight model systems were prepared for the research (see Table 1). The same residual petroleum oil was used as the dispersion medium for all model samples (see Table 2).

The protective properties of the eight model grease systems were evaluated under dynamic conditions, using the Dinacorrotest bench test to measure the corrosion rate of rolling friction bearings. In contrast to tests such as the DIN 51802 procedure and IP 220, which require167 hours to perform, time duration for the experiment in the Dinacorrotest instrument does not exceed three hours.

To measure corrosion, the Dinacorrotest instrument operates in two regimes: the first is for greases which are used in low-speed mechanisms with the speed of bearings rotation of 500 min-1; the second -for greases used in high-speed mechanisms -1,500 min-1.For this research, the protective properties of the eight studied model samples of sulfonate and alkyl salicylate greases were evaluated in the more severe regime, that is, under speeds of bearing rotation equal to 1,500 min-1.

Finally,the rate of the bearings corrosion was calculated on the basis of experimental values of polarization resistance. This resistance is measured with the universal corrosion rate meter.Calculation of corrosion rate is performed using the following formula:

in which: i cor =corrosion rate, A/cm2 ;K=recalculation constant of polarization resistance into the valueof corrosion rate,B(0.052); S =area of the electrode (10.6 cm2 ); I=measured current, A; E=polarization voltage, B(0.01); R ohm =ohmic resistance of the medium (e.g., the grease).

CLEAR DIFFERENCES

Figure 2 shows the values of corrosion rate, expressed as millimeters/year, of metal surfaces of bearings protected with sulfonate and alkyl salicylate model systems. (Note: For clarity,corrosion rate is very often given not in A/cm2 but in mm/year.)

As is obvious from this data, protective properties of alkyl salicylate grease are better than protective properties of sulfonate grease by orders of magnitude. The main reason lies in the varied nature of sulfo- and alkyl salicylic acids. Unbound with micellae, neutral calcium alkyl salicylate (CaAs2 )is more effective as a corrosion inhibitor in comparison with calcium sulfonate (CaSu2 )- which, being asulfuric compound, also has properties of a corrosion promoter.

Firstly,CaAs2 effectively inhibits processes of electrochemical corrosion, since stable, adsorbable and chemisorbable films formed with CaAs2 molecules inhibit the migration of ions of a metal into the volume of grease, which stimulates the concentrations anodic polarization and causes the initiation of more intensive polarization resistance under a layer of grease. Alkyl salicylate film also reduces the access of corrosion agents to a metal surface, lowering the metals cathodic corrosion.

Secondly,CaAs2 molecules are characterized with the high passivation ability that manifests itself in their interaction with ions of metal in the volume of grease and subsequent formation of chelate complexes.

ANSWERS ON THE SURFACE

In the MS-2 and MA-2 model systems -where CMB was simply added to the simple thixotropic grease -CMB penetrates into the micellaes nucleus and, in particular, loosens and increases their volume. This is partially due to the availability of polymeric chain an ions in its structure. As a result of this, a portion of CaSu2 or CaAs2 molecules are desorbed from the nucleus surfaces and transferred on the surface of metal – thereby increasing the density of the protective films.

In the MS-3 and MA-3 model systems, the presence of CHOS molecules (which, like the CaSu2 and CaAs2 molecules, are surfactants) contributes to the formation of denser,more protective films on the surface of metal. Such combined films provide effective protection of metal.

The most efficient protective properties of complex overbased greases can be associated with the formation of dense films consisting of individual surfactants and micellae on the surface of metal. Calcium carbonate from the micellaes nucleus participates in tribochemical transformations of surface layers. Corrosion-resistant ferritic structures (CaOFe 2 O 3 ) which are formed in the process possibly ensure the exceptional protective properties of these studied complex systems.