Water quality is important to the success of a water based metalworking fluid. This should not be surprising as a metalworking emulsion or solution in a machine tool will often be composed of between 3% to 10% fluid concentrate and therefore between 90% to 97% mix water.

So if we accept that the quality of the mix water will have an influence on a metalworking fluid, why do lubricant suppliers pay such little attention to it?

A simple test to do on site is to measure the water hardness (the level of dissolved salts) by the use of a water hardness stick. A one minute test which will give a basic indication along the lines of:

A high level of salts (hard water).

A medium level of salts.

A low level of salts (soft water).

With this basic information we can often make the following assumptions although at this point we do not know what salts are in solution and in what proportions:

A high level of salts – possible emulsion instability in the medium to long term and possible corrosion issues. Often any problems don’t immediately appear, they develop as the salts in the machine concentrate due to water evaporation.

A medium level of salts – usually ideal and recommended for most emulsions or solutions.

A low level of salts – the possibility of foaming especially in high pressure machines or machines with a high throughput of coolant. Unless of course the coolant has been developed to run on very soft water or demineralised water.

What can be done if the water quality dictates that there are potential problems ahead?

With the water being such an important constituent part of the emulsion or solution it would be nice to be able to dictate to the customer that they should do something about the water quality (usually softening the water at source with the use of a piece of capital equipment). This is good in concept but usually results in a conversation along the lines of ‘’other coolant suppliers tell me they can supply a product that does not require me to buy water softening equipment’’. This is usually not the reply you want to hear.

It is true that emulsions are developed to work with very hard waters. The emulsifier system is not readily affected by the build up of salts and the emulsion stays stable without the problems of either soap formation or oil splitting form the emulsion.

However, the build-up of salts can cause other issues. The emulsion may be stable and there may be no visible soap formation, but the corrosion protection performance has been compromised. This can often be seen by spot corrosion on machine beds or on tool holders and can be the point at which a metalworking fluid supplier indicates to the customer that the fluid needs to be changed. In other words, the life of the fluid is determined by its corrosion protection performance as opposed to bacterial degradation or gross contamination.

But what about synthetic solutions rather than emulsions? Surely we do not have to be so concerned about a synthetic solution because there can be no emulsion stability issues when you are working with a solution of chemicals rather than an oil based emulsion. Corrosion is often the downfall of a synthetic solution in hard water conditions. At least with an oil based emulsion there is usually a thin coating of oil on the machine tool to give some physical protection against corrosion. When the water phase of an emulsion evaporates the oil content remains but this is not the case with a synthetic solution. When the pH of a synthetic solution falls it will compromise the corrosion protection of that solution and with no physical oil present on surfaces, corrosion can be a real possibility.

So we know that water hardness plays an important part in the selection and performance of the metalworking fluid but what about other characteristics?

If the chosen mix water has a bacterial content this will immediately contaminate a fresh emulsion or solution, so will a water with an unusually high or low pH value. To simplify the discussion with the customer we need to state that the water used should be ‘drinking quality’ and therefore now have a bacterial content and have a pH close to neutral (7.0).

Realistically rain water from a roof tank containing a dead bird, is unlikely to a give a desirable mix water for water based metalworking fluids.