Coolant Formulation Relevant to Coolant System Cavitation
Coolant system cavitation in wet sleeve engine liners is just one aspect of coolant system technology trouble shooting. It is estimated that a majority of diesel engine downtime is coolant related and coolant leaks through wet sleeve liners is a major cause of coolant contamination in the engine lubricant. This is the first of several informational pieces on diesel engine coolant system reliability.
Cavitation in the coolant fluid on the outside of wet sleeve liners is caused by vibration of the cylinder wall while the piston is in motion. This vibration causes expansion and contraction of the cylinder sleeve, which in turn creates high and low pressure zones in the fluid between the cylinder sleeve and engine block where the engine coolant flows. During low pressure phases vapor bubbles are created. This results in cavitation when these vapor bubbles collapse violently (imploding) when the coolant fluid rapidly goes from a low pressure phase to a high pressure phase. These violent microscopic implosions at high pressures cause pitting and erode metal surfaces they are in contact with. Without inhibitor additives the process would quickly penetrate cylinder walls and cause detrimental harm to the engine lubricant and eventually the engine itself.
When using conventional coolants in a wet sleeve diesel engine liner application nitrite is the primary inhibitor to protect against cavitation erosion, which results in liner pitting on the outside of the cylinder sleeve. Some formulations will use molybdate in combination with nitrite for enhanced protection against cavitation. Nitrite will form a thin sacrificial protective oxide film on the coolant side of the liner wall. This oxide film quickly restores surface material over the liner by drawing additional available nitrite in the coolant.
Nitrites tend to deplete rapidly in conventional coolants and must be replenished at regular intervals with SCAs (Supplemental Coolant Additives). In order to replace the metal surface void and the displaced scale that was removed upon implosions due to cavitation, a cooling system using conventional coolant formulation must have remaining unconverted SCA in liquid form available in the coolant. The displaced scale from these systems becomes an abrasive particle floating around in the coolant, which requires periodic flushing. These abrasive scale particles can accelerate wear in cooling system components such as radiators, water pumps, and hoses.
Extended life fluids (ELCs) use organic acid technology (OATs) to protect wet sleeve cylinders from cavitation. Some organic acid formulations will include some levels of nitrites in their formulations also (NOATs). Where nitrite based inhibitors in conventional coolant formulations provide protection by forming a layer that coats or covers the total metal surface, organic acid inhibitors protect from cavitation by chemically interacting with the metallic surface itself. This eliminates the formation of sacrificial scale material characteristic of nitrite additives used in conventional coolants, which eventually has to be flushed out. Since the frequency required to flush coolant systems using organic acid inhibitors (OATs or NOATs) is greatly extended these formulations are referred to as ‘Extended Life Coolants (ELCs).
Currently engines from OEMs are factory filled with ELC coolants, unless otherwise specified from the customer. There is a push in the industry to go towards nitrite free ELC. Not all ELC coolant formulations are the same. The type and levels of organic acids can vary.
Whether a conventional coolant formulation is used or an ELC formulation is used these inhibitors do not inhibit or prevent cavitation, they inhibit the resulting erosion of the wet sleeve liner material in these cooling systems. Care should be taken to prevent improper mixing of conventional coolants and extended life coolants. These formulations are generally incompatible, which will cause improper coolant performance and damage to engine cooling systems.
ALS Tribology can test your diesel engine coolant for the proper level of additives for both conventional coolants and extended life coolant, or possible improper formulation mixing. SCAs can be monitored for proper levels. Too much SCA concentration can harm a coolant system as well. Wear metals such as iron along with chromium, nickel, silicon, or manganese, which indicate sleeve liner erosion, can be reported. The buildup of solids and particulate in the coolant, indicating scale buildup, can also be reported.
For further information feel free to contact ALS Tribology at any of our global locations or through our website.
David Doyle, CLS, OMA I, OMA II
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