Monitoring the degree of degradation of used oils

Acid number as an indicator of oil degradation

One of the indicators of oil degradation is the acid number (AN). This parameter is defined as the number of milligrams of potassium hydroxide needed to neutralise all acidic components present in 1 g of the tested lubricant (mgKOH/g). The acid number in fresh oils varies depending on the type of oil and depends on the chemical structure of the oil bases used and the composition of the additives, especially those containing zinc, e.g. ZDDP (anti-wear additive AW).

Oil degradation processes

In the initial phase of oil use, a slight decrease in the parameter value can be observed, related to the depletion/degradation of AW additives. Subsequently, due to factors such as atmospheric oxygen, temperature, light and the catalytic effect of metals, there is an increase in acid number, indicating the progressive ageing degradation of the lubricant. The increasing acidity of the oil is related to the formation of highly corrosive inorganic acids, e.g. sulphuric acid, and to chemical reactions leading to the formation of macromolecular organic acids. These are much weaker and less aggressive, but prone to agglomeration and condensation, and consequently to the formation of deposits.

i-pH as an acidity indicator

In everyday life, a commonly used indicator of the acidity of products such as food, cosmetics or household chemicals is the pH, which is a measure of the concentration of hydrogen ions H+. It should be noted that these products are aqueous solutions and, at the molecular level, differ significantly in polarity from oil bases. Therefore, the pH of an oil sample is not measured directly, but after dissolution in a standardised mixture of solvents. This measurement is referred to as i-pH.

Correlation between i-pH parameters and acid number allows estimation of the proportion of strong and weak acids in oil degradation products, enabling identification of risks to lubricated systems.

Base number in engine oils

Engine oils are supplied with rich packages of additives, including dispersant and detergent additives, whose task is to capture and neutralise acidic compounds generated during operation. As a result, the observed increase in acid number is insignificant. The content of active additives is determined by the base number (BN) parameter, defined as the number of milligrams of potassium hydroxide (KOH) corresponding to all alkaline components contained in 1 gram of the product, determined by titration with hydrochloric or perchloric acid solution.

The base number values of fresh oils vary significantly depending on the class and quality of the oil, for example from approximately 4 mgKOH/g for API CB class oils to over 15 mgKOH/g for engine oils exposed to significant loads of acidic combustion products, e.g. marine and railway oils. As the oil is used, a decrease in this parameter is observed, indicating the depletion of additives, known as a decrease in alkaline reserve.

Critical parameter values

Exceeding the critical level of acid number, base number or i-pH is a factor that disqualifies oil from further use. Acceptable limits vary depending on the type of oil and are not always obvious. The experience of a diagnostician is needed to interpret them correctly. As shown in Figure 1, regardless of the correct alkaline reserve, the i-pH value may be within the critical range, which indicates the need to determine this parameter.

Summary

In summary, regular engine oil analyses, within the appropriate testing range, ensure that the right operational decisions are made at the right time, which translates into correct planning and optimal use of the rolling stock. It is very important to use proven laboratory service providers who offer an appropriate analytical scope, have experienced and certified staff, and guarantee high reliability of test results thanks to laboratory accreditation.