Time for TBN (and all the rest)

TBN? What is that acronym? Is it on Google? Is it relevant for lubricant consumers to know? Why is TBN not displayed on a can of lube? Is TBN more relevant to specific operating conditions? What TBN should be present in oil and in which component is it critical? Is TBN available as an aftermarket additive? Who has ever heard of TBN retention? And then, in a truly relevant background is TAN which should not be overlooked.

TBN = Total Base Number and defined as ‘a property that measures oil’s ability to neutralize acids formed during engine operation.’ It is truly relevant for internal combustion engines.

In chemical terms, Total Base Number is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mgKOH/g). And TBN cannot be mixed in as an aftermarket additive – it must be present in the lubricant as part of the total additive package formulation.

TBN is an important measurement in petroleum products, and the value varies depending on its automotive or internal combustion engine application. Generally, oils with a higher TBN better neutralize acids and combustion by-products, resulting in longer oil life and improved protection against corrosion.

New engine oils usually have a TBN range of 5 to 15. As oil is used in service, it becomes contaminated with acids, causing the base number to drop over time. But this is dependent the fuel used, operating conditions and engine type. Below 3 is definitely time for an oil change.

The most common reasons for a TBN drop are related to low-quality fuel. During combustion, a low-quality fuel with high sulphur content can produce sulphuric acid, which needs to be neutralised, and the sacrificial TBN additive is consumed, reducing the available protection (TBN).

How is TBN measured – what is a recognised process? We went to the ‘coalface’ using internationally accepted standards. In this case, it is condition monitoring experts WearCheck whose Technical Manager, Steven Lumley, had the following to offer:

 

We have three methods of determining TBN

A titration is defined as ‘the process of determining the quantity of a substance A by adding measured increments of substance B, the titrant, with which it reacts until exact chemical equivalence is achieved (the equivalence point)’. From: Encyclopaedia of Analytical Science (Second Edition), 2005.

• ASTM D2896 (titration) – new oil,
• ASTM D4739 (titration) – used oil,
• FTIR (Fourier Transform Infrared spectroscopy) which predicts the TBN of the oil.”

Lumley continues – “TBN has traditionally been determined by chemical titration, but titrations are time-consuming and have a significant environmental impact as they produce a nasty waste stream due to the perchloric and hydrochloric acid used in the procedure. As a result, labs which perform a titration on engine samples must pass this cost on to the customer, and the large additional workload also slows down the turnaround time of the sample.”

“Even though titrations are accurate, says Lumley, “Many labs (like WearCheck) have adopted alternative technology to obtain base numbers, like FTIR (Fourier Transform Infrared) spectroscopy, which uses highly advanced mathematical models to determine a predicted base number. However, while cost-effective, the accuracy of the predicted results is dependent on the validity of the mathematical model and the presence of water and excessive soot in the sample i.e., if you have a badly degraded engine sample the FTIR can spit out rubbish.”

We adopted a different approach says Lumley – “We do the TBN by FTIR as standard on all engine samples and if the if the predicted TBN is lower than 6.0 or above 14 for commercial vehicle diesel engines, or if the sample contains water or a high amount of soot or is badly degraded, we then perform an actual TBN by ASTM D4739. All of this is automated and controlled by our lab software.”

The important thing about the two titrated methods is that if you use the wrong method, you get inaccurate results e.g., if you use ASTM D2896 (intended for use on new oils) on a used oil, the perchloric acid reacts with the wear metals in the used oil and gives an inflated TBN result. If you use ASTM D4739 (intended for use on a used oil) on a new oil, you will get an under estimation of TBN.”

Initial pH (i-pH) ASTM D7946

Lumley takes it further: “The i-pH value is considered an important parameter along with the TAN and TBN value, particularly for the evaluation of engine oils in biogas and landfill gas applications as it represents the strong acids in the oil which directly cause corrosion of engine components. Quantities of strong corrosive acids in oil, even if the TAN has not yet increased significantly”.

Lumley adds: “TAN provides an indication of acid concentration, but not acid strength. As such, it cannot always be relied upon to provide a reliable indication of the corrosion potential of an oil. To overcome this drawback, a new internationally applicable standard for determining the i-pH of an oil was adopted in June 2014 by the ASTM.”

This new method provides an absolute measurement of the corrosive potential of used oil and subsequent over-base additives depletion. The i-pH value is considered an important parameter along with the TAN and TBN value, particularly for the evaluation of engine oils in biogas and landfill gas applications as it represents the strong acids in the oil which directly cause corrosion of engine components. While TAN and TBN provide information on the overall content of acidic or alkaline compounds respectively, the I-pH value allows the acidity to be qualitatively assessed.

This method can even be used to detect minor quantities of strong corrosive acids in oil, even if the TAN has not yet increased significantly.

The i-pH and TAN test methods measure distinct aspects of an oil’s acidic or alkaline character. The i-pH test method measures the apparent pH of the oil. The apparent pH is a representation of how corrosive the oil may be, but it does not indicate the concentration of acidic or alkaline constituents.

The pH test method is useful in applications where corrosive oil could cause considerable damage. It is also valuable in lubricant systems with a high potential for the formation or the contamination of strong acids.

The last word goes to LUBRIZOL in a technical summary offered at the conclusion of a recent webinar. Lubrizol360 Webinar Series: Measuring Lubricant Lifetime featuring Mathew Robin, PhD & David Growney, PhD

• Inclusion of i-pH in used oil analysis ensures integrity of vehicle engine lubricants
• Traditionally TAN and TBN have been used to measure acid and base in engine lubricant – an important consideration for engine durability
• TBN is not a sufficient indicator of a lubricant’s ability to protect the engine against corrosion due to the complexity of neutralisation
• TBN and TAN measurements are not an effective way to measure the build-up of harmful corrosive acids as the market adoption to ultra-low Sulphur fuels increases
• According to LUBRIZOL i-pH measurements give the best indicator of corrosion risk

Clearly TBN is not a measurement that must be carried in isolation of TAN and i-PH. An integrated view and trend-line are essential. The process of assessing engine lubricant life must be subject to detailed and disciplined SOPs to extract the value that condition monitoring offers.

A company fleet tribology policy is way beyond the simple factors of price, brand and viscosity rating and it is time to upgrade – even if there is an automotive lube policy in place.