oil-related corrosive wear

[moribundman]

Primus recently brought up the issue of corrosive wear, which may well be related to the kind of oil used. If corrosive wear may indeed make up 30% of bearing and/or ring wear, then I think this topic deserves investigation. Since I have no knowledge to offer in this matter, I hope someone else does.

 

[MolaKule]

Well, first of all, I don't agree with Primus' postulates.

Wear is caused by mechanical loads exceeding the pressure characteristics of the oil's hydrodynamic film layer and additive AW capabilities, while corrosion is a chemical process.

Corrosion is caused by a number of factors, including acid buildup from sludge. In modern day oils with their anticorrosion, antirust, and metal deactivators, I cannot fathom that much corrosion from any oil, assuming the drain interval does not exceed the capabilities of the oil.

 

[Robbie Alexander]

quote:

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Originally posted by MolaKule:
assuming the drain interval does not exceed the capabilities of the oil.

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Yeah... Keep in mind, there are thousands of oils.
Every oil will have their strength... and weakness. Part of the weakness or strength is how well it fights the acids, and they all do it differently, although some slight...
And Yes, IMO to some extent the film strength and everything else may have some-thing to-do with the chemical attacks on the oil.
It is my belief, with the advancements in chemical additives and such, that the chemical breakdowns have more to do with wear than Mechanical loads.

 

[slalom44]

I agree with Robbie. My gut tells me that chemical attack is more significant than we would like to believe. When you turn your engine off, the rings stop at a point on the cylinder wall. Between the wall and the ring is a "crevice condition" which it ideal for chemical attack.

Unlike the oil in your oilpan, the oil in the ring/wall interface is high in moisture due to combustion. Add the sulfur and free carbon that has accumulated in the oil from previous combustions, and you have yourself a weak sulfuric and carbonic acid.

The acid neutralizers in the oil do their best to neutralize these acids but if the acid formation in that "microclimate" exceeds the neutralizing cabability (ala TBN), then you get oxidation (corrosion). When you start your engine, you wipe off that layer of oxide. It may be a small layer but over time, those layers translate into reduced thickness of the rings and cylinder walls.

Short vehicle trips result in more moisture trapped in the combustion chamber. This means more water, thus more corrosion. We know that short trips increase the water content of oil in the oilpan. Imagine what it does locally at the cylidner/ring interface!

BTW, I'm a Metallurgical Engineer, and I have a fair amount of experience with crevice corrosion and corrosive wear.

 

[Drstressor]

Slalom44 explained the process well and Primus brings up a very interesting point that I have also wondered about. It takes much lower levels of oxidation inhibitors to stabilize a PAO or Group III oil than an older Group I base stock. So when an engine sits idle for some period of time, atmospheric oxygen should deplete the available anti-oxidants faster with a synthetic oil than with an older conventional oil. With moisture present, corrosive acids can form. I am referring to reactions in the thin film of oil remaining on drained metal surfaces, not in the crankcase. I'm just speculating here, but didn't Bob once show something like this with a "rusty nail" test?

 

[mikemc]

This is just one more reason I favor dino oil and relatively short (3-4K) drain intervals. The only data I have to back this up is several vehicles that ran great with no smoking, loss of power or oil consumption after I put a lot of miles on them.

 

[Jay]

I was thinking of the instance where the additive package of the oil itself causes corrosive wear, such as certain GL-5 oils corroding soft metals in a transmission where GL-4 should have gone.

 

[MolaKule]

quote:

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The acid neutralizers in the oil do their best to neutralize these acids but if the acid formation in that "microclimate" exceeds the neutralizing cabability (ala TBN), then you get oxidation (corrosion). When you start your engine, you wipe off that layer of oxide. It may be a small layer but over time, those layers translate into reduced thickness of the rings and cylinder walls.

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What are we talking about here, a 5 nanometer layer of "possible" oxidation? Your ZDDP, MoTDC, Boron, etc, AW and antioxidant adds create a thicker layer than that.

Jay,

The older GL5's stained and cause corrosion on copper alloy metals such as brass synchronizers.

 

[Primus]

quote:

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Originally posted by MolaKule:

quote:

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The acid neutralizers in the oil do their best to neutralize these acids but if the acid formation in that "microclimate" exceeds the neutralizing cabability (ala TBN), then you get oxidation (corrosion). When you start your engine, you wipe off that layer of oxide. It may be a small layer but over time, those layers translate into reduced thickness of the rings and cylinder walls.

---

What are we talking about here, a 5 nanometer layer of "possible" oxidation? Your ZDDP, MoTDC, Boron, etc, AW and antioxidant adds create a thicker layer than that.

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Apparently yes, the process is continuos, so wiping off the layer of oxide leds to problems. As for MoTDS, it high content causes serious corrosion of bronze where appearance of Cummins test L-38.

 

[slalom44]

quote:

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Originally posted by MolaKule:
What are we talking about here, a 5 nanometer layer of "possible" oxidation? Your ZDDP, MoTDC, Boron, etc, AW and antioxidant adds create a thicker layer than that./I]

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I don't know the thickness, but your point is well taken. It isn't much. But if moisture is greater at the rings when the engine sits idle and the oil film is mostly squeezed out (the rings are compressed by the cylinder wall and they are not moving), then the little oil that is left is being taxed heavily.

Here's an analogy: In the coal handling industry, coal chutes are often made from steel plate. The coal slurry (mixed with water to get it to move) causes very high wear rates, and the chutes wear out quickly even though they show minimal signs of corrosion. When switched to a low grade stainless steel (similar to type 409 used in auto exhaust systems), the wear was reduced by an order of magnitude. However lab tests show that the actual wear rates of these two metals is very similar. The difference is attributed to "corrosive wear".

Granted, coal slurry is hardly the same as a lubricated piston in a cylinder, but my point is that metal oxides wear much faster than metals. And besides, metal oxides are abrasives.

 

[Drstressor]

quote:

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What are we talking about here, a 5 nanometer layer of "possible" oxidation? Your ZDDP, MoTDC, Boron, etc, AW and antioxidant adds create a thicker layer than that.

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Once these compounds become oxidized (in the presence of O2 and H20) they become pro-oxidants, which attack the ferrous material and soft metals. That's why the concentration of up-stream electron donors (anti-oxidants)is critical when an engine sits idle for long periods.

 

[kev99sl]

quote:

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Apparently yes, the process is continuos, so wiping off the layer of oxide leds to problems. As for MoTDS, it high content causes serious corrosion of bronze where appearance of Cummins test L-38.

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So what you're insinuating is that the high PPM we see with soft metals in Red Line analyses stems from their use of Moly?

 

[MolaKule]

"Apparently yes, the process is continuos, so wiping off the layer of oxide leds to problems. As for MoTDS, it high content causes serious corrosion of bronze where appearance of Cummins test L-38. "

Wiping off an oxide layer exposes the metal to antirust, and metal deactivator additives. I did a test on a Kohler K341 last winter that sits outside all the time and is only operational occasionally. I took M1 10w30 SS and then added some extra rust Inhibitor to bring it up to the same level as an HD diesel oil's level. I have yet to oberve any rust on the cylinder walls.

Just to clarify, MoDTS was "suspected" as a 'possible' cause but never proven unequivocally. We have had two prior discussions on acid attack and MoDTS already.

MoDTS is not used in any diesel or gas engine oil, while MoDTC, a more stable form, is used in both. Schaeffer's has used various forms of moly in their HDD oils for many years without corrosion or acid attack problems resulting from moly.

All is not lost, MoDTS is an excellent anti-oxidant, FM, and Anti-Wear agent for AW Hydraulic oils.

[ January 08, 2004, 06:03 PM: Message edited by: MolaKule ]

 

[Primus]

quote:

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Originally posted by MolaKule:
Just to clarify, MoDTS was "suspected" as a 'possible' cause but never proven unequivocally. We have had two prior discussions on acid attack and MoDTS already.

MoDTS is not used in any diesel or gas engine oil, while MoDTC, a more stable form, is used in both. Schaeffer's has used various forms of moly in their HDD oils for many years without corrosion or acid attack problems resulting from moly.

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Sorry for my misprints in the wording "As for MoTDS, it high content causes serious corrosion of bronze where appearance of Cummins test L-38". Correct should be "... MoDTP ... Cummins test in addition to L-38".

Unfortunately I don't have links to the presentation of J. A. Mc Geehan and P. R. Ryason "Million Mile Bearings: Lessons From Diesel Engine Bearing Failure Analysis". This presentation was made at SAE meeting in Toronto and their study on bearing failure is still considered as one of the most complete nowadays.

 

[Primus]

quote:

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Originally posted by kev99sl:

So what you're insinuating is that the high PPM we see with soft metals in Red Line analyses stems from their use of Moly?

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Naturally not, the question was about base oil. In my post concerning Red Line I expressed my supposition that one of the reasons of non-impressive UOAs with RL could be its base stock and namely esters, i.e. what is perfect for racing does not usually mean good for everyday use. Hear probably I have to duplicate my original post in order to avoid some misunderstanding:

"I am afraid that higher wear numbers for ester oils come from higher corrosive/oxidative wear. It is noticed that mineral and semi-synthetic oils usually exibit lower corrosivity then fully synthetic oils, especially for relatively short intervals 7.000-9.000 km. Then the difference in corrosive wear starts to decrease and to 15.000 km synthetic oils would have only slightly worse or equal numbers with mineral or semi-synthetic oils.

If to compare synthetic oils, according to tests run by our car magazine ester based oils showed considerably higher corrosivity then oils formulated mainly with PAO. To mesure weight loss of metal plates due to corrosion at 9.000 and 15.000 km it was used high temperature oxidation test (1 hour is considered as about 3.000 km). In the same table you will find shear stability. Unfortunately the procedure was not decribed: I could find only that they applied a higher temperature then 100 C (possible they used the same CEC L-14-A-93 with over 100 C and over 30 cycles).

...................................... Corrosion, g ........... Shear stability, %
Motul 300V 5W-30 ............. 7,0 ... 17,8 ............ - 9,0 ... - 3,0
Motul 8100 0W-40 ...................... 16,0 ....................... - 43,3
Shell Helix Ultra 0W-40 .................. 1,8 ....................... - 26,9
Mobil1 0W-40 ............................. 12,0 ...................... - 32,6
Castrol RS 0W-40 ......................... 7,0 ....................... - 49,5
Liqui Moly Synth. 5W-40 ................ 7,1 ....................... - 23,1
Chevron Delo-400 5W-40 ............. 10,0 ....................... - 40,0
Shell Helix Plus 10W-40 ....... 0,1 .... 8,2 .......... - 14,0 ... - 24,0
Castrol GTX5 10W-40 ......... 0,7 .... 3,3 .......... - 11,0 ... - 28,0
BP Visco 3000 10W-40 ........ 0,3 .... 6,0 ........... + 7,2 .... - 4,0
Valvoline Dura Blend ........... 3,7 ... 10,5 .......... - 25,0 ... - 27,0
Esso Ultra 10W-40 ............. 3,2 ... 11,0 .......... - 24,0 ... - 13,0
Castrol GTD 10W-40 ........... 2,0 .... 9,0 .......... - 10,0 ... - 29,0
Shell Helix Super 10W-40 .... 3,2 ... 12,0 .......... - 18,0 ... - 13,0
Liqui Moly Tour. 10W-40 ..... 5,6 ... 19,0 .......... + 12,0 ... + 41,0 "

 

[Primus]

According to many studies just a corrosive-oxidative wear, and not abrasive wear, is one of the main causes of engine failure, especially in diesel engine. Acids from combustion, moisture, water or coolant in oil, low quality gas, oxygen access to oil free metal parts, all this things are greatest contributors to engine wear.

Undoubtedly corrosion inhibitors prevent from corrosion and rust, but their ability to do it differs from one oil to another and we could see this from mentionned test. And if we are ready to consider this property of oils and its contribution to the wear, no lab's comments that all these oils meet API, ACEA or OEMs criterias would not be enough. For me this test results discovered 1). chemical reactivity of synthetic oils is higher then that of mineral or semi-synthetics and, in this respect, synthetics don't give an advantage over mineral oils for short intervals, 2). certain synthetics provide far less anticorrosion protection then others.

 

[yannis]

It looks like 10W40's are not that bad in tearms of shear stability after all.

 

[Leo]

Look at 300V's shear stability. Incredible really. It is truely world class stuff but the corrossion due ot the ester base has got me thinking.

No doubt its the old formulation, as the "Double Ester" generation has replaced it in the past 2 years. They claim 0% shear with the new one.