Calcium, magnesium and TBN

[Danh]

The answer to this question may be obvious to some, but I wanted to see if my understanding was correct.

As I understand it, calcium and magesium are typically the additives used for acid neutralization, and high levels of these components tend to correlate with a high beginning TBNs. But when looking at UOAs of oils that have much-depleted TBNs, the levels of calcium and magnesium are not that different from fresh oil.

I assume this is because when, for example, calcium interacts with hydrochloric acid it forms calcium chloride and the tests run by Blackstone and others still detect the calcium. In other words, the calcium is still present, but not in a form that will allow it to neutralize more acid. Does this seem about right?

Thanks.

 

[jorton]

I thought acid neutralization was a function of antioxidant, ZDP or ZDDP but I really don't know if that's correct. And I thought calcium and magnesium are detergent-AW additives.

 

[zpinch]

Magnesium is a cheaper and lesser quality additive vs Calcium, calcium is much better for TBN retention. Zinc, when used up, it turned into a TBN contributor, also.

 

[Garak]

I think you better check on that. Magnesium packages are more expensive and do have extremely good TBN retention. Sodium packages tend to be cheaper, but there is no concern over its efficacy.

 

[zpinch]

Let me rephrase that, a Mg sulfonate detergent does not neutralize acidic oxidation products as effectively as Ca phenate detergent. Some studies have shown Mg based detergents result in increase bearing corrosion because of this. It doesn't neutralize acids, hence the appearance of a higher TBN. I prefer an oil with loads of Calcium.

 

[Garak]

That part may be true, but that has nothing to do with costs, which are higher with Mg, and there are reasons to choose Mg over Ca.

 

[weasley]

Originally Posted By: Danh
I assume this is because when, for example, calcium interacts with hydrochloric acid it forms calcium chloride and the tests run by Blackstone and others still detect the calcium. In other words, the calcium is still present, but not in a form that will allow it to neutralize more acid. Does this seem about right?


That's pretty much it. The normal acid-neutralising species are alkaline salts of calcium or magnesium, let's say calcium carbonate, CaCO3. Calcium carbonate is, essentially, chalk which is insoluble in oil, so it is 'carried' into the oil by the detergent molecules, let's say calcium sulfonate. The detergent molecules surround the 'chalk' and form small micelles (around, say, 10µm in diameter) which are dispersed throughout the oil. When the alkaline CaCO3 meets a droplet of acid, let's say sulfuric acid (since you are more likely to find a sulfur-derived acid than a chlorine-derived one in an engine) you get the classic "acid + base = salt + water" reaction, so:

CaCO3 + H2SO4 --> CaSO4 + H2O + CO2

As you have worked out, the Ca is still present, and the detergent molecules will still carry it (and the water) around the oil until it is changed, so you will still see Ca in UOA. You can't destroy the elements or change them to another element, all you can do is change what molecules they are part of. In this case, the CaSO4 salt is much less harmful to the engine than the H2SO4.

 

[MolaKule]

That was a very good explanation weasly.

Back when I was just a kid...
QOTD: Detergents and Dispersants