Cutting Fluid Maintenance: pH, Microbiology and Tramp Oil Control

A water-mix cutting fluid is a living system from the moment it is filled into the machine: water evaporates, oil leaks in, swarf accumulates and micro-organisms use the emulsion as a nutrient. This guide covers the three control parameters that determine fluid life: pH, microbial load and tramp oil. Measurement methods, threshold values and the order of intervention are set out for each. Mix preparation and product selection are dealt with separately in the cutting fluid types and concentration and mix preparation guides.

1. The mechanism of deterioration

An emulsion consists of water, a mineral or ester based oil, an emulsifier, a corrosion inhibitor and an alkaline buffer. Most of these components are a direct carbon source for bacteria and fungi. As the micro-organisms multiply they consume the emulsifier and the corrosion inhibitor; the organic acids that appear neutralise the alkaline buffer. The result progresses as a chain: pH falls, corrosion protection weakens, the emulsion separates and odour develops.

The conditions that accelerate growth are well defined. Low concentration, tramp oil collecting on the surface, fine swarf left at the bottom of the sump, a fluid temperature that rises appreciably above ambient temperature and stagnant zones (dead ends, little-used lines) can raise the microbial load in a short time. HSE recommends that sections without flow be removed at the system design stage, that they be flushed weekly where they cannot be removed, and that cooling be considered where the fluid temperature rises appreciably above ambient temperature.

Cutting fluid maintenance is not a chemical dosing task but a cleaning and flow management task. Chemical interventions made without mechanical measures do not give a lasting result.

2. pH: working range and the meaning of a fall

For the majority of water-mix products the working range is given as pH 8.4–9.2. Below the lower limit both corrosion protection weakens and an environment favourable to microbial growth forms. Above the upper limit, the risk of dark staining on aluminium and aluminium alloys, excessive foaming and skin irritation increases.

The trend is more meaningful than the absolute value of a single measurement. Where the weekly records show a one-directional and continuous fall, the fact that the value on the day of measurement is still inside the range is misleading. In Blaser Swisslube's publications, the principal causes of a sharp fall in pH are given as low emulsion concentration and high tramp oil ingress; a sudden rise in pH, on the other hand, is mostly the sign of contamination by an alkaline cleaner.

In pH measurement there is a practical difference between a strip (indicator paper) and a digital pH meter. The strip is fast and cheap, but the colour of the emulsion and the oil film reduce the accuracy of the reading; it does not show small changes reliably. Where a trend is to be tracked, a calibrated digital pH meter is preferred.

3. Microbiology: bacteria, fungi and biofilm

Three groups of organism are found in the system. Aerobic bacteria multiply rapidly in an oxygenated environment and are the group that first breaks down the emulsion. Anaerobic bacteria, sulphate-reducing ones in particular, come into play once the oxygen is exhausted and produce hydrogen sulphide (H₂S), creating the rotten-egg smell. Fungi and yeasts consume the corrosion inhibitors, cause a musty smell and form gelatinous plugs in hoses, filters and nozzles.

The biofilm that builds up on sump surfaces is far more resistant than free-floating cells. The cells inside a biofilm are markedly protected against biocide; for this reason chemical addition alone gives no result in a system where biofilm has formed, and mechanical cleaning becomes mandatory. The sludge that collects at the bottom of the sump is likewise an oxygen-free and nutrient-rich growth bed.

The Monday morning smell

When the machines stop over the weekend, circulation is interrupted. The tramp oil film on the surface blocks oxygen transfer, the dissolved oxygen is exhausted within a few hours and the anaerobic population becomes dominant. The sharp smell noticed when the cover is opened on Monday morning is the escape of hydrogen sulphide produced over the weekend. The odour is not an aesthetic problem but a direct indicator that the system has been left without oxygen.

4. Measurement programme and threshold values

HSE's MW5 guidance note defines a workable monitoring schedule for water-mix fluids. The table below summarises this schedule, the measuring instruments and the relevant reference values; the pH range comes from the product data sheet and the nitrite limit from TRGS 611.

A dip slide is a paddle coated with a different culture medium on each face: one face is designed for general bacteria, the other for fungi and yeasts. The paddle is immersed in the fluid, the excess is drained off, it is incubated in a closed tube, and the density of the colonies that grow is read on a logarithmic scale by comparison with the manufacturer's chart. The sump has to be stirred before the sample is taken; otherwise the tramp oil on the surface coats the paddle and makes the result read low. HSE recommends that all measurement results be kept for at least five years and be entered in a table or chart so that the trend can be seen.

5. Tramp oil: source, effect and separation

Tramp oil is oil that enters the system from outside and is not part of the emulsion. Its principal sources are slideway and way oils, hydraulic oil, gearbox oil and the protective oils carried in on the workpiece. It cuts off oxygen transfer by forming a thin film on the surface; this is the strongest trigger of anaerobic growth. It also consumes the emulsifier, destabilises the emulsion, increases mist formation and causes dip slide results to be read incorrectly.

Cutting off the source is always the cheapest solution: reviewing the way oil feed frequency, renewing the sealing elements and keeping the workpiece from entering the machine oily reduce tramp oil ingress directly.

6. Swarf, fines and filtration

Coarse swarf collects at the bottom of the sump, both loading the pump and forming an oxygen-free bed. Fines, on the other hand, remain in suspension, have an abrasive effect at the cutting edge and reduce surface quality. HSE describes the removal of fines by centrifuge, filtration systems or, on ferrous materials, magnetic separators, and the removal of coarse swarf with suitable tools such as brushes and shovels, with the machine stopped and with suitable gloves. Removing swarf by hand is not recommended because of the risk of cuts.

Regular checking of filters and strainers for blockage is as important for microbial control as it is for flow continuity: the stagnant volume behind a blocked filter is an ideal zone for biofilm. An excessive load of fines also shortens tool life; the mechanism of this effect is covered in detail in the tool wear mechanisms guide.

7. Aeration and shutdown management

Oxygen is the cheapest chemical against anaerobic growth. Maintaining circulation both renews the dissolved oxygen and prevents stagnant zones from forming. During long shutdowns, running the pump several times a day on a timer is a common practice. Aeration by bubbling compressed air is also used; however, since a high air flow rate increases foaming and mist formation, it has to be kept moderate.

Aerosol exposure: As mist and aerosol formation is increased, the respiratory risk increases with it. No common occupational exposure limit value is defined in the EU for metalworking fluid aerosol as a whole. In the USA, the OSHA permissible exposure limit (PEL) for mineral oil mist is 5 mg/m³ and the NIOSH recommended limit (REL) is 5 mg/m³ as a time-weighted average with 10 mg/m³ as a short-term value; the ACGIH threshold limit value for mineral oil, on the other hand, excludes metalworking fluids from its scope. Ventilation and mist control therefore have to be planned together; the subject is dealt with separately in the occupational health and safety in metalworking fluids guide.

8. Biocide use and its limits

A biocide is not a product that replaces the maintenance programme but the final step of that programme. HSE guidance states explicitly that biocide is not to be added before concentration, pH, tramp oil content, metal contamination, operating temperature, mixing and flow have all been corrected. If growth continues despite these corrections, it has to be added only at the dose given by the supplier and at a point of high turbulence. Overdosing increases skin sensitisation and leads to the selection of a resistant population.

The regulatory side is decisive as well. In the EU, biocidal products fall under Biocidal Products Regulation No 528/2012; in the regulation's Annex V list, metalworking fluid preservatives appear under Product Type 13 ("working or cutting fluid preservatives"). Its counterpart in Türkiye is the Biyosidal Ürünler Yönetmeliği (the Turkish Biocidal Products Regulation), published in the Official Gazette No. 27449 (4th Repeated) of 31/12/2009; placing an unauthorised product on the market and using it is not permissible. In addition, the registration and notification of chemicals is assessed under the KKDİK Yönetmeliği (the Turkish REACH equivalent) and workplace obligations under the İş Sağlığı ve Güvenliği Kanunu No. 6331 (the Turkish Occupational Health and Safety Law).

Formaldehyde-releasing biocides (formaldehyde donors) are a topic of their own. By Commission Regulation No 605/2014, formaldehyde has been classified in the harmonised list in CLP Annex VI as carcinogen category 1B (H350) and mutagen category 2; the same entry gives a specific classification limit of 0.2% for skin sensitisation, and these provisions have applied since 1 April 2015. The classification and labelling obligation arising from the released formaldehyde has pushed the sector away from preservatives of this kind. Two tendencies are seen in practice: alternative preservatives that do not release formaldehyde, and products formulated to be bioresistant from the outset that require no tank-side additive. In product selection, the classification in the safety data sheet has to be checked.

9. Sump cleaning and system disinfection

Cleaning the system does not consist merely of draining the fluid and filling with new. As long as the biofilm remains on the surfaces, the fresh fluid is contaminated again within a week. The sequence defined in HSE's MW3 guidance note is as follows:

1. System cleaner fluid is added and circulated for the period stated by the supplier.
2. The fluid–cleaner mixture is drained; the remaining fluid and swarf are cleaned out mechanically.
3. Hose connections, filters, splash guards and tramp oil separators are cleaned.
4. If the system is heavily contaminated, a second cleaner is filled in warm water, circulated for the stated period and drained again.
5. Fresh fluid is filled at the concentration recommended by the supplier.
6. A measurement is taken with a dip slide one week later; if the result is below 10,000 cfu/ml the system continues in use, otherwise the cycle is repeated.

The use of a high-pressure water hose during cleaning has to be avoided as far as possible; the mist it creates increases the respiratory risk. In operations that create mist, respiratory protection of at least APF 20 class (FFP3 or powered TH2) has to be used, and in large enclosed tanks confined space working procedures have to be applied. The drained fluid and cleaning wastes are treated as hazardous waste; the disposal rules are covered in the corrosion inhibitors and waste disposal guide.

10. Symptom – cause – solution table

11. The cost effect of fluid life

The purchase price of a cutting fluid is a small part of the total cost of ownership. A sump that deteriorates early comes back as the cost of hazardous waste disposal, the working hours spent draining and cleaning the sump, machine downtime, increased biocide consumption, scrap caused by corrosion and shortened tool life.

Regular monitoring reduces these costs in two ways. First, weekly records catch problems at a stage where they can still be corrected; the cost difference between a measure taken at the 10,000 cfu/ml level and the full drain that becomes mandatory at the 1,000,000 cfu/ml level is large. Second, in systems where records are kept, fluid replacement is tied to measurement rather than to the calendar.

A workable programme consists of three items: a one-page weekly record sheet for each machine, keeping the skimmer and filtration in continuous operation, and ensuring circulation during shutdowns. The options on the product side can be reviewed in the chemicals category.

Sources

• HSE — COSHH essentials for machining with metalworking fluids, MW5: Managing fluid quality (2021). hse.gov.uk/pubns/guidance/mw5.pdf
• HSE — COSHH essentials for machining with metalworking fluids, MW3: Sump cleaning — water-mix fluids (2021). hse.gov.uk/pubns/guidance/mw3.pdf
• OSHA — Permissible Exposure Limits: Annotated Table Z-1 (comparison of the OSHA PEL, NIOSH REL and ACGIH TLV for mineral oil mist). osha.gov/annotated-pels/table-z-1
• Regulation (EU) No 528/2012 concerning the making available on the market and use of biocidal products — Annex V, Product Type 13 (Working or cutting fluid preservatives). eur-lex.europa.eu
• Commission Regulation (EU) No 605/2014 — the formaldehyde entry in CLP Annex VI (Carc. 1B, Muta. 2; specific classification limit of 0.2% for skin sensitisation). eur-lex.europa.eu
• ASTM E2275-24 — Standard Practice for Evaluating Water-Miscible Metalworking Fluid Bioresistance and Antimicrobial Pesticide Performance. astm.org
• Blaser Swisslube — Monitoring pH value and emulsion hardness (blaser.com) and Nitrite and microbial growth — TRGS 611 nitrite upper limit (blaser.com).
• Biyosidal Ürünler Yönetmeliği (the Turkish Biocidal Products Regulation) — Official Gazette, 31/12/2009, No. 27449 (4th Repeated). resmigazete.gov.tr