Degreasing and Metal Surface Preparation Before Coating

Why a Coating Lasts for Years or Peels Off in a Week

The quality of any coating — paint, powder enamel, galvanic layer, adhesive joint — is 80% determined by surface preparation. Not by the paint composition, not by the layer thickness, not by the application technology. But by what was on the metal before coating.

A fat film 0.1 µm thick — invisible to the eye — reduces paint adhesion by 40–60%. MWF residues after metalworking, fingerprints after manual operations, a microlayer of oxides after warehouse storage — all these are barriers between the coating and the metal. The result is blistering, bubbling, peeling, and under-film corrosion.

When one of our clients switched to powder coating, we transitioned them from trichloroethylene degreasing to an aqueous alkaline process — coating adhesion even improved, and ventilation costs were reduced by 60%.

Degreasing is the first and critical step in the surface preparation chain. A mistake at this stage cascades and ruins all subsequent operations: phosphating will not form a uniform film, the paint will apply with defects, and the galvanic coating will have pores.

What Exactly Needs to Be Removed from the Surface

Before choosing a degreasing method, it is important to understand what contaminants we are dealing with. They are fundamentally different in chemical nature — and require different approaches.

Oils and Lubricants

The most common type of contamination in metalworking enterprises. Sources:

• MWF (metalworking fluids) — remain on parts after turning, milling, and drilling operations. Water-soluble emulsions wash off easier than mineral oils. More on choosing MWF — in the guide on MWF for CNC machines.
• Stamping oils — thick, viscous, often with sulfur- or chlorine-containing additives. One of the most difficult types of contaminants to degrease.
• Preservation lubricants — applied to protect against corrosion during storage and transportation.
• Fingerprints — seem like a minor detail, but the skin's sebaceous secretions contain salts and organic acids that provoke local corrosion.

Oxide Layers

Metal oxidizes in the air. A thin layer of oxides (5–20 nm on aluminum, 10–100 nm on steel) forms in minutes. Thicker layers — rust (Fe₂O₃), scale after heat treatment, mill scale — require mechanical or acid treatment, not just degreasing.

Dust and Abrasive Particles

Metal dust from grinding, abrasive residues after sandblasting, atmospheric dust during storage. These particles get under the coating and create peeling points.

Old Coatings and Residues

During repair painting or reprocessing, the surface may have residues of previous coatings, fluxes after soldering, or marking paints. Removing them requires either specialized strippers or more aggressive degreasing modes.

Industrial Degreasing Methods

Alkaline Degreasing

The most common industrial method. The base of the solution is sodium hydroxide (NaOH), silicates, phosphates, carbonates, surfactants (surface-active agents). Working pH: 10–13.

Mechanism of action: the alkaline solution saponifies fats (converts them into water-soluble soaps), and surfactants emulsify mineral oils — enveloping oil drops with micelles and keeping them in solution. Silicates and phosphates bind water hardness ions, preventing the precipitation of insoluble salts on the surface.

Typical parameters:

• Concentration: 20–50 g/l
• Temperature: 50–80 °C
• Processing time: 3–10 minutes (immersion), 1–3 minutes (spray washing)

Advantages: high efficiency against most industrial contaminants, low cost of chemicals, ease of control, minimal environmental impact.

Limitations: aggressive to aluminum, zinc, and tin at pH > 12 — corrosion inhibitors or special formulas are required. Does not remove mill scale and rust. Definition of technical terms — in the industrial chemistry glossary.

Acid Degreasing

Performs a dual function: removes both fat contaminants and oxide layers (rust, scale) in one stage. Base — phosphoric acid, corrosion inhibitors, surfactants. pH: 1–4.

Mechanism of action: the acid dissolves metal oxides, and surfactants in an acidic environment degrease the surface. Phosphoric acid additionally forms a thin phosphate film — temporary corrosion protection.

Typical parameters:

• Concentration: 30–80 g/l
• Temperature: 20–50 °C (acidic agents work at a lower temperature than alkaline ones)
• Processing time: 5–15 minutes

Advantages: simultaneous degreasing and oxide removal, works at a lower temperature (energy savings), surface passivation.

Limitations: possible metal etching if overexposed, aggressive to non-ferrous metals, requires thorough rinsing to remove acid residues.

Solvent Degreasing

Used for heavy contaminants that aqueous solutions cannot handle: polymerized oils, wax coatings, bitumen mastics, adhesives. More about types of detergents — in the article «Industrial Detergents: Types and Selection».

Main technologies:

• Vapor degreasing. Parts are suspended over a boiling solvent (perchloroethylene, modified alcohols). Vapors condense on the cold surface of the part, dissolve the contaminants, and drain off. Advantages — minimal solvent consumption, perfectly clean surface. Equipment — closed systems with condensers for vapor recovery.
• Ultrasonic degreasing. Parts are immersed in a bath with a solvent or aqueous solution. Ultrasonic generators (frequency 25–40 kHz) create cavitation bubbles that implode near the part's surface and mechanically tear off contaminants. Effective for complex geometry: blind holes, threaded connections, internal channels.

Limitations: environmental risks (chlorinated solvents are restricted by regulatory norms), fire hazard (hydrocarbon solvents), higher disposal costs.

Electrochemical Degreasing

The highest level of surface cleanliness. The part is immersed in an alkaline electrolyte and connected as a cathode or anode. When a direct current (5–15 A/dm²) passes through, gas bubbles (hydrogen at the cathode, oxygen at the anode) are released on the surface, mechanically tearing off residual contaminants.

Typical parameters:

• Electrolyte: NaOH 30–50 g/l + surfactants
• Temperature: 40–70 °C
• Current density: 5–15 A/dm²
• Time: 1–3 minutes

Advantages: removes even monomolecular fat films, provides perfect preparation for electroplating and precision coatings.

Limitations: requires special equipment (rectifier, current leads), not suitable for large parts, cathodic mode can cause hydrogen embrittlement of steel.

Comparison of Degreasing Methods

Key Parameters of the Degreasing Process

Degreasing efficiency is determined by five parameters. Changing any of them affects the result.

Temperature

Rule: increasing the temperature by every 10 °C doubles the chemical reaction rate. For alkaline degreasing, the optimal range is 60–70 °C. Below 40 °C — surfactants lose efficiency, oil viscosity increases, emulsification slows down. Above 80 °C — excessive evaporation, risk of contaminants burning onto the surface, increased energy costs without a proportional gain in quality.

Concentration

Underestimated concentration — insufficient degreasing. Overestimated — overconsumption of chemistry, formation of a residual film on the surface, increased water consumption for rinsing. The optimum is determined by laboratory testing on a specific type of contaminant. Control — titration or refractometry.

Contact Time

The minimum required time for complete degreasing depends on the thickness and type of contamination. Stamping oils — 5–10 minutes, light MWF — 2–3 minutes, fingerprints — 1 minute. Overexposure during acid degreasing threatens metal etching.

Mechanical Action

Spray washing (pressure 1–3 bar) is 3–5 times more effective than immersion at the same concentration. The reason is the mechanical tearing off of contaminants and the constant renewal of the solution near the part's surface. Ultrasound additionally increases efficiency by 5–10 times due to cavitation.

Rinse Water Quality

After degreasing — mandatory rinsing. Water quality is critical: hardness above 5 °dH leaves calcium and magnesium salts on the surface, which degrade coating adhesion. For critical manufacturing, deionized water (conductivity < 20 µS/cm) is used at the final rinsing stage.

Degreasing Quality Control

Degreased — and how to make sure the surface is really clean?

Water-break test

The simplest and most common method (ISO 12981). Water is applied to the cleaned surface: a clean surface holds a continuous water film without breaks for 30 seconds. If the water beads up or the film breaks — fat remains on the surface.

Advantages: free, instant result, requires no equipment. Limitations: qualitative method (clean / dirty), does not measure the degree of contamination quantitatively.

Contact Angle Measurement

Quantitative method: a goniometer records the angle between a water drop and the surface. A perfectly clean surface — angle < 5°. Acceptable level for painting — < 15°. Angle > 30° — insufficient degreasing. The method is more accurate than the water-break test but requires a special device (goniometer).

UV Control

Many industrial oils fluoresce under an ultraviolet lamp (wavelength 365 nm). The method allows detecting local contaminant residues invisible under normal lighting.

Environmental Aspects

Industrial degreasing generates wastewater containing oils, surfactants, alkalis, or acids. Modern trends:

Solvent-free formulas. Replacing chlorinated and hydrocarbon solvents with aqueous alkaline agents. Reducing VOC (volatile organic compounds) emissions by 90%+. Regulatory pressure — EU Directive 2010/75/EU (industrial emissions).

Extending bath service life. Modern degreasers with oil skimmer systems operate for 3–6 months without replacement. Collected oils are sent for regeneration, not into drains.

Closed-loop rinsing. Cascade rinsing (3 baths in series) reduces clean water consumption by 5–10 times. Deionization allows water reuse.

Biodegradable surfactants. Transition to surfactants with biodegradability > 90% in 28 days (criterion OECD Test Guideline 301). Reduces the load on wastewater treatment plants.

Common Mistakes in Degreasing

1. Using Household Products

"Fairy washes grease" — yes, at 40 °C and on a plate. Industrial contamination — mineral oils with additives, stamping oils with fillers — requires specialized formulas with industrial surfactants, controlled foaming, and corrosion inhibitors. More details — in the article «Industrial Detergents: Types and Selection».

2. Degreasing "by Eye"

Lack of quality control after degreasing is a direct cause of mass coating defects. The water-break test takes 10 seconds and costs zero hryvnias. Ignoring it is like not checking the dimensions of a part after machining.

3. Ignoring Rinse Water Quality

A perfectly degreased surface, rinsed with hard water (> 15 °dH), receives a microlayer of carbonates. Adhesion decreases, and the source of the problem is sought in the paint, in the degreaser — everywhere except the rinse.

4. One Formula for Everything

A degreaser for ferrous metals can etch aluminum in 5 minutes. An acidic agent for steel can destroy a zinc coating. Each material and type of contamination requires a separate selection.

5. Skipping Degreasing After Inter-Operational Storage

Parts were degreased, placed on a rack, and sent for painting 3 days later. During this time — condensation, dust, fingerprints. Degreasing must be performed immediately before applying the coating, with a minimal pause between operations.

FAQ

How Does Alkaline Degreasing Differ from Acid Degreasing?

Alkaline degreasing (pH 10–13) effectively removes oils, lubricants, and organic contaminants through saponification and emulsification. Acidic (pH 1–4) additionally dissolves metal oxides — rust and scale. The choice depends on the type of contamination: if there is only oil on the part — alkaline. If oil + rust — acidic or a sequence of alkaline → acidic. For aluminum, zinc, and tin, an alkaline solution with pH > 12 is dangerous — special low-alkaline formulas or neutral degreasers are required.

What Is the Optimal Temperature for Industrial Degreasing?

For alkaline degreasers — 60–70 °C. Below 40 °C, efficiency drops sharply: surfactants work worse, oil viscosity increases. Above 80 °C — unnecessary energy consumption without a significant improvement in the result, plus the risk of evaporation. Acidic agents are effective even at 20–40 °C, which provides 30–40% savings on heating. Electrochemical degreasing — 40–70 °C. The exact temperature is selected by testing on a specific type of contamination.

How Often Should the Solution in the Degreasing Bath Be Changed?

Depends on the intensity of the load and the type of contaminants. The average service life of an alkaline solution is 2–4 weeks under medium load. Signs of degradation: decrease in washing ability (fails the water-break test), increase in oil content (> 10–15 g/l — visually noticeable oiliness), pH change by 1–2 units from the working one. Systems with oil skimmers extend the service life by 2–3 times, removing floating oil from the bath surface.

Can Aluminum Be Degreased with a Standard Alkaline Agent?

No. Aluminum is an amphoteric metal: it dissolves in both acids and alkalis. A standard alkaline degreaser with pH > 12 causes aluminum etching (dissolution with hydrogen evolution). For aluminum, low-alkaline formulas (pH 9–11) with corrosion inhibitors (silicates) or neutral degreasers (pH 7–9) based on non-ionic surfactants are used. Processing time is also reduced — 2–5 minutes instead of 5–10 for steel.

SVK Degreasers

SVK manufactures industrial degreasers for all types of metalworking enterprises: alkaline formulas for general degreasing of ferrous metals, low-alkaline for aluminum and non-ferrous alloys, acidic for simultaneous degreasing and rust removal.

Each product is developed for a specific task: degreasing before painting, before welding, before electroplating, before gluing. Formulas are compatible with spray washers, immersion baths, and ultrasonic units.

"Test Drive" Program: we send samples of your profile — you test on your parts, with your contaminants, on your equipment. The result is a test protocol with recommendations on concentration, temperature, and processing time. We do not recommend a product without testing under real conditions.

Order a degreaser test drive →

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Read Also

• Phosphating vs. Nanoceramics: Technology Comparison — the next step after degreasing: choosing a conversion coating technology
• How to Choose MWF for a CNC Machine — the right MWF simplifies subsequent parts degreasing
• Industrial Detergents: Types and Selection — complete classification of industrial detergents by chemical type
• Industrial Chemistry Glossary — terms and definitions: surfactants, saponification, cavitation, adhesion