Corrosion Inhibitors, Protective Oils and Waste Disposal: Corrosion Protection in Machining

In machining, corrosion mostly arises not in the cutting operation itself but in the waiting, storage and shipment stages that follow it. This article covers the classification of temporary corrosion protection products, the working principle of VCI packaging, the standards by which protection performance is measured, and how used machining fluids are disposed of under Turkish legislation. Maintenance of the cutting fluid during service is a separate subject and is dealt with in the cutting fluid maintenance guide.

1. The mechanism of corrosion and the moments it appears

Atmospheric corrosion is an electrochemical phenomenon. At the anodic region of the metal surface the metal ionises, at the cathodic region oxygen is reduced, and these two reactions are completed by ions conducted through the thin water film on the surface. Accordingly, corrosion starts when three components are present at the same time: water, oxygen and dissolved salts that raise conductivity. Chloride and sulfate ions accelerate the effect both because they raise conductivity and because they locally break down the protective oxide film.

In machining, the situations where the risk rises are clear:

• Interoperation waiting. The part leaving the machine is still hot; as it cools, condensation forms on its surface. When the cutting fluid film dries and thins, protection disappears, but the salt residue on the surface remains.
• Humid environment with condensation. In environments where the relative humidity rises above roughly 60%, the water film on ferrous surfaces becomes continuous. On cold mornings, warming workshop air creates direct condensation on the cold part surface.
• Cast iron chips. In graphite-bearing cast iron, the graphite behaves as a cathode and the ferrite matrix as an anode. A pile of wet chips creates rapid staining on the sheet or table surface beneath it. For this reason cast iron chips are chosen as the reference material in corrosion tests.
• Mixed metals. The presence of metals with different electrode potentials, such as aluminium with steel or copper with aluminium, in the same fluid or the same packaging leads to galvanic corrosion. Deposition of copper ions on an aluminium surface is a typical form of damage.

Corrosion and concentration: Corrosion is often directly related to the concentration of the fluid. In water-mix fluids, a concentration falling below the recommended range causes the corrosion inhibiting additive to become ineffective. Correct mix preparation is explained in the concentration guide.

2. Temporary corrosion protection products

Temporary protection products are classified along two separate axes: the carrier fluid and the film they leave on the surface. The carrier determines how the product is applied, how fast it dries, and the occupational health and fire risks. The film, in turn, determines the protection period and how the part will enter subsequent operations.

The choice of film type depends on where the part is going. A dry film is preferred for parts going straight into assembly, and an oily or waxy film for semi-finished parts that will stay in the store for months.

The periods given in the table are typical ranges stated in manufacturers' technical documents; the actual period varies markedly with ambient humidity, temperature fluctuation, handling frequency and surface cleanliness. For solvent-based water-displacing protectives, for example, manufacturer data states indoor protection of up to twelve months; the same product lasts a far shorter time outdoors.

3. VCI paper and bags

VCI (vapour corrosion inhibitor) products are compounds loaded into paper, film or an emitter that evaporate slowly at room temperature. The active substances are generally the salt of a volatile amine and an organic acid. The evaporating molecule reaches equilibrium in a closed volume, condenses on the metal surface and forms a protective layer of the order of a single molecule. This layer suppresses corrosion by slowing the anodic and cathodic reactions.

Three practical consequences follow from the working principle:

1. A closed volume is essential. Leaving the bag open or a leaking box leads to a fall in vapour concentration and to the end of protection.
2. No line of sight is required. The vapour also reaches holes, channels and internal surfaces that are not in direct contact. On parts with complex geometry this is an advantage over dip oil protection.
3. Metal compatibility has to be confirmed. VCI technology was initially developed for ferrous metals, and some amine derivatives cause staining on copper, brass and aluminium. For shipments containing mixed metals, a product declared as "multimetal" has to be chosen. In current formulations, compositions free of nitrite, phosphate, chromium and heavy metals are common; the nitrite content has to be confirmed, since it is expressly prohibited in some customer specifications.

VCI is used as a complement to protective oil as much as an alternative to it. Placing a part that has been given an oily film into a VCI bag provides a second layer of defence should the packaging be damaged at one point.

4. Storage and shipment packaging

Three factors are decisive in packaging: closure, moisture load and mechanical contact. Free water or a damp wooden pallet entering closed packaging consumes VCI protection in a short time. On long sea shipments, the use of a desiccant together with a barrier film is common; in that case the aim is to keep the relative humidity inside the packaging low.

Handling machined surfaces with bare hands is also a known source of damage. Since sweat contains chloride and organic acid, local corrosion starts in the fingerprint area. For this reason parts to which protection has been applied have to be handled with clean gloves.

5. Test methods

Corrosion protection performance is assessed not by declaration but by standard tests. On the machining side, two different test families are used: chip tests that measure the corrosion protection of water-mix machining fluids, and cabinet tests that measure protective films and coatings.

The expression of the IP 287 result as "the lowest concentration giving a stain-free result" is important in practice: this value is used directly in setting the lower concentration limit to be maintained in the plant. The salt spray test, on the other hand, is accepted as not directly predicting real atmospheric life, and as being used for comparison between products and for detecting discontinuities.

6. Part cleanliness and the chloride problem

A protective film does not give the expected performance on a dirty surface. Chip dust, salt residue and oxidised oil remaining on the surface form a local corrosion nucleus beneath the film. For this reason a cleaning step before the protection is applied is regarded as mandatory.

• Alkaline cleaners. These are water-based systems containing surfactants and alkaline salts. They effectively remove oil and emulsion residues. On amphoteric metals such as aluminium and zinc, high-pH formulations lead to dulling of the surface, so low-alkaline or neutral products are preferred for these metals.
• Solvents. They dry quickly and leave no water. Against this, they require ventilation and personal protective equipment because of the flash point, the vapour concentration and the exposure limits. Chlorinated solvents are used less and less, both for health reasons and because of the chloride problem below. Exposure limits and personal protective equipment requirements are set according to the values stated in the safety data sheet of the product used.
• The requirement to leave no chloride on stainless steel. Austenitic stainless steels are susceptible to chloride stress corrosion cracking. Grades 304 and 316 are clearly sensitive to this mechanism; the risk rises as temperature, chloride concentration, low pH and tensile stress increase. For stainless parts, therefore, a cleaner and a protective with low chloride content have to be chosen, and rinsing with deionised water after cleaning is required. Other aspects of machining stainless are dealt with in the ISO M milling guide.

7. Waste management of used emulsion

Used water-mix cutting fluid is one of the largest-volume hazardous waste items accumulating in a plant. In Turkey, classification is made according to the Annex-4 waste list of the Atık Yönetimi Yönetmeliği (the Turkish Waste Management Regulation). Wastes arising from the physical and mechanical surface treatment of metals and plastics are collected in group 12 01; starred codes indicate hazardous waste.

The final assignment of the code is made according to the actual constituents of the waste and its halogen content; in case of uncertainty, the analysis result and the opinion of the competent authority have to be relied upon.

Separation methods

The greater part of the volume of used emulsion is water. Since disposal cost is proportional to volume, separating the water phase directly lowers cost. Three methods are common:

• Ultrafiltration. The membrane retains emulsified oil and high molecular weight components; water, dissolved salts and small organic molecules pass through as permeate. The concentrate phase is shipped as hazardous waste.
• Chemical splitting. The emulsion is destabilised with an acid, salt or polymer based splitter (demulsifier); the oil phase rises to the surface and is skimmed off. It is generally applied as a pretreatment before ultrafiltration or evaporation.
• Evaporation. In a vacuum evaporator the water phase is removed at low temperature, leaving a small volume of concentrate. Energy consumption is high, but it is the method with the highest volume reduction ratio.

The literature reports significant reductions in chemical oxygen demand with ultrafiltration and vacuum evaporation applied after a splitting pretreatment. This does not mean that the separated water phase can be discharged directly; for discharge, the relevant treatment and permit conditions have to be met.

Handover and record obligations

Hazardous waste may be handed over only to recovery or disposal facilities licensed by the Ministry of Environment, Urbanisation and Climate Change, and only through a licensed carrier. The transport operation is recorded through the Mobile Hazardous Waste Tracking System (MoTAT) within the Integrated Environmental Information System (EÇBS); the transport number corresponding to the National Waste Transport Form (UATF) is obtained through this system. In addition, the waste producer has to submit the annual waste declaration through the system and to keep the transport documents for the period laid down in the legislation. The practical requirements applied on site are as follows:

• Separate collection of the waste at source and no mixing of hazardous with non-hazardous waste.
• A temporary storage area with an impermeable floor, a covered top and secondary containment.
• Labelling of the containers with the waste code and hazard markings.
• Safety data sheets (SDS) kept accessible on site.

8. Fluid recovery from oily chips

Chips leaving the machine carry fluid amounting to a significant proportion of their weight. This fluid creates three separate losses at once: loss of purchased fluid, sale of the scrap chips at a low price, and disposal of oil-containing metallic sludge as hazardous waste. Centrifugal chip wringer equipment separates the fluid by spinning the chips at high rotational speed. Equipment manufacturers report that, depending on the application, the greater part of the fluid on the chips can be recovered. Briquetting presses, in turn, compress the chips, both separating the fluid and turning the scrap into a high-density briquette.

Returning the recovered fluid directly to the system is not always appropriate. Since the fluid may carry fine metal dust, tramp oil and a bacterial load, it has to be fed back after filtration and, if necessary, concentration correction. Approaches that reduce fluid consumption at source are dealt with in the MQL and dry machining guide. ATH Tools chemical products can be examined in the chemicals category.

Sources

• ISO 9227:2022 — Corrosion tests in artificial atmospheres — Salt spray tests. iso.org
• ISO 6270-2:2025 — Paints and varnishes — Determination of resistance to humidity — Part 2: Condensation (in-cabinet exposure with heated water reservoir).
• DIN 51360-1 / DIN 51360-2 — Testing of cooling lubricants; determination of corrosion preventing characteristics of cooling lubricants mixed with water (Part 1: Herbert corrosion test; Part 2: chip/filter paper method).
• ASTM D4627-22 — Standard Test Method for Iron Chip Corrosion for Water-Miscible Metalworking Fluids. astm.org
• Energy Institute, IP 287:2008 — Determination of rust prevention characteristics of water mix metalworking fluids — Cast iron drillings/filter paper method.
• T.C. Çevre, Şehircilik ve İklim Değişikliği Bakanlığı — Atık Yönetimi Yönetmeliği (RG 2.4.2015 / 29314), Ek-4 Atık Listesi. mevzuat.gov.tr
• HSE (UK) / UKLA — Good Practice Guide for Safe Handling and Disposal of Metalworking Fluids. ukla.org.uk
• Specialty Steel Industry of North America — Chloride Stress Corrosion Cracking. ssina.com