TI100 - Pretreatment of Metals

1. Goals of metal pretreatment

The most important goals of any organic coating regarding the workpiece to be coated include:

  • Uniform powder coating film
  • Good adhesion to the metal substrate
  • High resistance to creeping corrosion

To improve corrosion resistance and adhesion as well as to ensure uniform coating, the surfaces must be pretreated.

This is usually carried out chemically (Chapter 3) or mechanically (Chapter 4) to ensure the substrate is clean and to achieve correct adhesion.

2. Test methods

Ultimately, the success of chemical pretreatment is revealed by the long-term performance of the coated workpiece in practical use. In terms of checking, selecting, and developing pretreatment methods, there are a number of test methods that quickly deliver conclusive answers. The most important ones for rapid corrosion testing are the salt-spray test (DIN EN ISO 9227) and tests in a condensation-water atmosphere (DIN EN ISO 6270-2).
In the salt-spray test, inadequate pretreatment is revealed by excessive corrosion product infiltration of the coating, starting from a scratch point. The combination of stress types in alternating tests leads to corrosion effects which are largely similar to those caused by open-air weather exposure.

The evaluation of the corrosion effects is standardized in DIN EN ISO 4628-8 (infiltration), DIN EN ISO 4628-3 (rust), and DIN EN ISO 4628-2 (blisters).

Powder adhesion without corrosive stress is tested by means of a:

  • mandrel bending test, cylindrical mandrel (DIN EN ISO 1519)
  • impact resistance test (ISO 6272 / ASTM D 2794)
  • cupping test (Erichsen cupping EN 5020)
  • cross-cut adhesion test (DIN EN ISO 2409)

3. Chemical pretreatment

The purposes of chemical pretreatment methods are:

  • Removal of damaging substances from the surface, e.g., scale, rust, abrasion products, grease, oil, dust.
  • Creation of a coat that promotes powder adhesion and inhibits corrosion, e.g., by means of phosphating, chromating, chromium-free methods, etc.
  • Removal of damaging treatment substances from previous process stages by means of thorough rinsing.

3.1. Surface cleaning

Any contamination that remains on the metal surface before coating will impair adhesion between the powder coating and the metal. This is why contaminants such as grease, oil, sanding dust, rust, and scale must be removed. You can use alkaline or acid cleaning agents to remove oils, greases, and dirt.

To support the cleaning performance, you can apply higher temperatures or active baths as well as mechanical force, in particular spraying or ultrasound treatment.

As a rule, steel can be cleaned using highly alkaline agents, while galvanized steels and aluminum must be treated with mild alkaline cleaners or in a mild acidic medium.

When cleaning zinc or die-cast aluminum, light pickling of the   metal surface may be unwanted, even though this actually increases the cleaning effect. The pickling effect can be reduced by adding special additives to the cleaner. Rust and scale are removed by pickling in acids. The acids usually contain inhibitors that prevent dissolution of the bare metal. Usually, scale is removed from unalloyed steel by means of sulfuric or hydrochloric acid pickling. The preferred method of rust removal from these surfaces is using inhibited phosphoric acid. Iron and steel materials are often cleaned in an electrolytic process supported by high-alkaline cleaning concentrates. Stainless and alloyed steels are usually pickled with a pickling solution containing nitric acid and hydrofluoric acid.

Pickling solutions containing surfactants which remove rust and grease in one go are used for parts which are only lightly contaminated with easy-to-remove oil. If there are no special corrosion-protection requirements for the workpiece, subsequent phosphating is not necessary after pickling with phosphoric acid. This deposits a thin, bluish and iridescent phosphate film on the surface which provides a good adhesive base for subsequent coating. It also acts as temporary corrosion protection.

After pickling and alkaline cleaning, the parts must without fail be rinsed with water.

3.2. Creating conversion layers by phosphating

Conversion layers are created by chemical reaction of the metal surface with the treatment solution, which leads to an inseparable, usually inorganic layer.

3.2.1. Alkaline phosphating

In alkaline or iron phosphating, the metal surface reacts with an acidic solution of alkaline phosphates. These watery solutions of phosphate ions do not contain their own cations that are involved in the layer formation. The cations for the layer formation come from the substrate material. This is why alkaline phosphating is often termed non-layer-forming phosphating. (See also 3.2.2. Zinc phosphating)

The layer produced by alkaline phosphating on ferrous metals is an amorphous agglomerate of phosphates, oxides, and hydroxides of bivalent and trivalent iron with a layer weight of 0.2 – 1.0 g/m², which is equivalent to a layer thickness of around 0.15 – 0.8 μm.

Depending on the sheet metal quality, layer thickness, and accelerator used, the phosphated surfaces can display colors ranging from yellow to iridescent blue, gold, or gray.

Alkaline phosphating can also be effective for other metals such as zinc and aluminum alloys.

Generally, alkaline phosphating is not sufficient for coated surfaces permanently exposed to weather and humidity. However, it is suitable as corrosion protection for powder coated parts in unstressed indoor environments.

3.2.2. Zinc phosphating

In contrast to alkaline phosphating, phosphating with an acidic solution of primary zinc phosphate is layer-forming and produces layer thicknesses of around 8 – 20 μm. With this method, zinc or metal ions from the phosphate solution provide the layer-forming cations, while phosphate from the phosphor solution acts as the anion.
Oxidation agents accelerate the layer formation of tertiary zinc and zinc-iron phosphate (phosphophyllite, Zn2Fe(PO4)2) on the steel surface.

After further oxidation, the iron dissolved during layer formation separates out as iron phosphate. Due to the chemical reactions from layer formation as well as the discharge with the workpieces, the phosphating solution loses active components. These are continuously replaced by adding a supplementary solution to the bath.

To create fine-crystalline layers with optimal properties, the workpieces are often pre-rinsed before phosphating with activating agents – usually on the basis of titanium compounds. These activating agents are also often integrated in the alkaline cleaning agent so that no additional treatment stage is necessary. Phosphating procedures on the basis of zinc phosphate are used extensively for pretreatment for a wide range of organic coating systems.

Pretreatment in the form of zinc phosphating produces the best corrosion resistance for coatings on steel as well as cadmium-plated, and galvanized steels. The adhesion of the organic coating under bending and impact stresses also meets stringent requirements. Special solutions for aluminum and other alloys are possible, but they require their own baths.

3.2.2. Zinc phosphating

Phosphating solutions for certain purposes also contain other substances, for example nickel or manganese for the treatment of zinc surfaces, or fluorides for aluminum.

When galvanized surfaces are phosphated, no iron ions are available for targeted inclusion in the phosphate layer. Therefore suitable cations such as nickel, manganese, and calcium are added to create phosphating solutions that match the corrosion protection effect of phosphophyllite layers. As these phosphating solutions usually contain zinc, manganese, and nickel as layer-forming cations, this process is also termed “trication”. In this context, processes without nickel are termed “dication”.

Complex fluorides are frequently added to achieve an even layer formation on hot-dip galvanized surfaces, or on steel surfaces which are difficult to phosphate. The addition of fluorides forms complexes of aluminum ions which usually dissolve in the acidic phosphating bath as part of the galvanization process.

3.2.4. Posttreatment of zinc-phosphated surfaces

Posttreatment is an additional option to increase the corrosion protection effect of phosphate layers.

Today, chrome-free post-passivation agents are usually used which seal the phosphate layer by closing open pores in the layer. A distinction is made here between organic and inorganic products.

Organic passivation agents contain polymers with complex-forming properties, while inorganic products contain complex zirconium or titanium fluorides which form insoluble phosphates on the surface.

3.2.5. Posttreatment of galvanized surfaces prior to duplexation

Even before coating, the quality of the galvanization should be discussed with the galvanizing company. Galvanizing companies often recommend posttreatment in accordance with DIN EN ISO 1461-2009 (Hot dip galvanized coatings on fabricated iron and steel articles (batch galvanizing); specifications and test methods). This inhibits white rust formation and also homogenizes and extends the gloss of the zinc surface.

The experience of IGP Pulvertechnik AG has shown that this kind of posttreatment is usually detrimental to good intercoat adhesion of powder coatings. Therefore tests should be performed on the material to be coated prior to coating.

3.2.6. Zinc phosphating prior to cathodic electrodeposition coating

The introduction of cathodic electrodeposition coating has led to a significant increase in corrosion protection after subsequent powder coating. However, in order to take full advantage of this solution, newer zinc phosphating methods can be used. These are characterized by low zinc and high phosphate contents. When sprayed onto steel, they produce layers of hopeit (zinc phosphate Zn3(PO4)2) and phosphophyllite (zinc phosphate (Zn2FePO4)2). In the immersion method, they produce layers consisting mainly of phosphophyllite.
Due to the lower zinc content, the process time is usually slightly longer than for the conventional methods.

3.3. Creation of conversion layers by pretreatment with chromium

3.3.1. Situation of pretreatment methods with chromium

The disadvantages of the methods of yellow and green chromatizing described below result from the multiple hazard potentials of chromium(VI) oxide CrO3, which is a very good oxidation substance and not only flammable but also highly toxic, carcinogenic, and mutagenic. The chromatizing takes place in watery baths containing, among other substances, chromic acid (dissolved CrO3). For the above reasons, these baths are highly water-polluting. This applies both for transparent chromatizing and for the two chromatizing methods with greater layer weights: yellow and green chromatizing.

Although only the chromium(VI) compounds are significant in toxicological and mutagenic terms, green chromatizing is increasingly under examination even though the layers formed after the reaction theoretically consist of non-toxic chromium phosphates (CrPO4) and aluminum phosphates (AlPO4). This is due firstly to the fact that chromium(VI) compounds are always used to produce the treatment chemicals for yellow and green chromatizing. Secondly, the possible residual share of hexavalent chromium with a green chromatizing layer of around 0.01 μg/cm² is usually below the detection limit.

Even if the use of pretreatments containing chromium is currently not banned by national regulations, it is advisable when choosing pretreatments containing chromium for façade components to check the regional regulations as well as the performance specifications in the invitation to tender. This applies especially in the case of public or publicly funded construction projects.

3.3.2. Yellow and green chromatizing

Yellow and green chromatizing processes are distinguished by the color of the conversion layers created at greater layer thicknesses. Both treatment types can be implemented in spray or bath processes.

Treatment solutions for yellow chromatizing contain hydrofluoric acid, chromic acid, possibly other additives, and accelerators. As a result of the primary aluminum dissolution caused by the acids, layers of the mixed oxides of aluminum and the trivalent and hexavalent chromium are formed on the aluminum surface. Generally, the layer weights produced by yellow chromatizing are 400–1000 mg/m².

Green chromatizing solutions essentially contain hydrofluoric acid, chromium acid, and phosphoric acid. Therefore, as with yellow chromatizing, the layer formation reaction also produces chromium trioxide (CrO3) or chromium(VI) oxide.

The fluoride concentration determines the layer weight. If the layer weight is not too high, the layer is free from hexavalent chromium. However, it cannot be ruled out that, especially in the case of structures which, due to their recesses or cavities, may “scoop up” thicker layers which retain chromates from the bath solutions. These can dry on after rinsing and are potentially hazardous in many respects, plus they impair adhesion.
The chromate layer removed consists of phosphates of the aluminum and the trivalent chromium and does not have a crystalline structure.

For use as a pretreatment before coating, weights per unit area of 400 to 1200 mg/m² are applied.

Both yellow and green chromatizing achieve an excellent improvement in adhesion and corrosion inhibition for the subsequently applied coating. Often, yellow or green chromatizing is used to create conversion layers on galvanized steels for subsequent organic powder coating.

Chromatizing is also used without any further organic coatings as a corrosion protection for bare metals. In special cases, yellow or green chromatizing layers can be used for decorative purposes. The layer weights in this case are higher, at 1 to 3 g/m².


3.3.3. Passivation solutions containing chromium(III)

Despite the chromium basis, chromium(III) passivation can also be considered a more environmentally friendly method than yellow and green chromatizing (chromiumVI compounds).
Trivalent chromium forms reaction products (chromium(III) oxides) on the aluminum surface that are not readily soluble. These are good corrosion inhibitors and enable good powder coating adhesion. Trivalent chromium has long been used for the passivation of zinc and zinc alloys, and is known as blue passivation. A few manufacturers have recently introduced pretreatment chemicals and/or processes for the chromium(III) passivation method on aluminum which are certified by the quality associations Qualicoat and GSB. On aluminum, they mostly form slightly iridescent layers in a shade depending on the alloy.
Before conversion formation, the surface must be free of grease and oxides. This can be achieved by
a) acidic pickling or
b) alkaline (pickling) degreasing and acidic etching.
The surfaces must be thoroughly rinsed between the individual passivation stages, and as far as possible soft water should be used before the passivation bath. In order to ensure optimal corrosion protection, purified water should be used for the final rinse after passivation.

3.4. Formation of conversion layers with chromium-free pretreatments

3.4.1. Non-alloy layer components

Above all, the significantly lower environmental impact as well as lower costs for occupational health and environmental protection are driving the move away from chromium to chromium-free pretreatment.
As a rule, the bath procedure and analysis require slightly more care and the rinsing processes are slightly more work-intensive.
The corrosion protection for possible interim storage is usually weaker.

There are a number of suppliers of chromium-free pretreatment chemicals. Based on the chemicals used, they are distinguished as follows:
a) Titanium and/or zirconium compounds
b) Titanium/(fluorine) polymer compounds
c) Zirconium/fluorine compounds
d) Organosilanes
Most chromium-free pretreatments are also suitable for spray or dip processes. Some are multi-metal-compatible. They differ with regard to the necessity for rinsing after conversion coating (rinse or no-rinse methods).

We recommend choosing pretreatment processes which have been approved by the quality associations GSB and/or Qualicoat. This verifies that they meet the requirements for long-term storage (e.g., Hoek van Holland) as well as many requirements relating to process stability and application suitability. Similar to pretreatments involving chromium, the substrates must first be cleaned and rinsed. Acidic cleaning is sufficient for some chromium-free pretreatments of certain steels and galvanized steels. Aluminum substrates must undergo degreasing by means of pickling. Usually, degreasing must be followed by several rinsing processes with purified water. With no-rinse methods, there is no need for rinsing after conversion treatment. Both in the rinse and the no-rinse methods, chromium-free processes require higher drying temperatures. With open-pore substrates, the higher temperatures offer the advantage that they promote exhalation prior to coating.

4. Mechanical pretreatment

Apart from the above wet chemical processes, mechanical cleaning processes or pretreatment methods can be used, above all for unalloyed, low-alloyed, and galvanized steel. Mechanical treatment can achieve various aims:

  • Removal of grease, contamination, and corrosion products such as rust and scale.
  • Removal of welding residues.
  • Breaking up of sharp lasered and cut edges.
  • Increasing the surface area for good coating adhesion, especially on ridges, edges, and interfaces.

For more information, see “DIN 55633: Paints and varnishes – Corrosion protection of steel structures by powder coating systems – Assessment of powder coating systems and execution of coating”, Chapter 6 Surface preparation.

4.1. Blasting steel

When processing steel, the complete removal of rust down to the bare metal through mechanical brushing, grinding, or blasting is one of the preconditions for achieving a corrosion-resistant coating. In each case, untreated steel surfaces must comply with the surface preparation grade Sa 2½ in accordance with DIN 55928 T4.
Mechanical roughening significantly improves the adhesion of the coating to the substrate. Suitable blasting agents are mineral or silicate agents such as corundum and glass.

As a rule of thumb, the more sharp-edged and larger the blasting material, the rougher the surface. This is associated with better adhesion of the primer on the substrate, which in turn leads to better corrosion protection. Rounded grain is less abrasive and compacts the material detrimentally. To ensure good paint adhesion, the average surface roughness Rz attained should be between 40 μm and 80 μm.

4.2. Sweep blasting of galvanized steel

The existing corrosion protection layer (e.g., electro-galvanization or strip galvanization) must not be damaged by the surface preparation.

The process known as sweep blasting defined by DIN EN ISO 12944-4 is a procedure very similar to compressed-air blasting. The main differences to the latter are the much lower pressure (2.5–3 bar) and the type of blasting agent. The blasting agent is finer (particle size 0.25 mm–0.5 mm) and must not contain any corroding metallic components. Non-metallic blasting agents as defined by DIN EN ISO 11126-3 to DIN EN ISO 11126-7 have proved their effectiveness as blasting agents, as well as metallic agents such as chromium casting granulate (grit) or glass (grit).

Welding beads and scale should be removed using a grinder, as far as the profile geometry allows. If necessary, a blasting gun and mineral blasting agent (e.g., corundum) can be used to finish off the job.

It should be pointed out here that in addition to the pretreatment of galvanized steel using sweep-blasting under Item 6.2.3, DIN 55633 also permits wet chemical pretreatment in the form of yellow-chromatizing with a layer weight of 0.5 g/m² to 1.0 g/m². In the event of any deviations, and if other equally suitable pretreatment methods are used, these should be agreed separately.

4.3. Blasting steel

The mechanically treated surfaces are highly oxidative due to the increase in surface area and must be subjected to additional processing without delay.

Firstly, the blasting material and contaminants must be removed using compressed air and/or chemical cleaning or chemical pretreatment. Compressed air usually only removes coarse dust while, for example, chemical surface treatment such as iron (Fe) phosphating not only removes the finest dust but also offers a certain degree of corrosion protection for steel surfaces during further treatment.

Chemical surface treatment is a useful supplement to mechanical pretreatment for the subsequent powder coating and promotes adhesion on the substrate:

  • For steel substrates, phosphating processes are stipulated in accordance with DIN EN9717:2013-07 “Phosphate conversion coatings of metals – Method of specifying requirements” – preferably zinc phosphating (see Chapter 3.2). Chromatizing is not possible for untreated steel.
  • For galvanized steels, following mechanical pretreatment by sweep blasting, in addition to zinc phosphating, chromatizing in conformity with DIN EN12487 is beneficial.

Zirconium and/or titanium-based chromium-free conversion layers can achieve equivalent results to conventional chromatizing.

We recommend obtaining proof of the required adhesion values by means of corrosion protection tests. (E.g., varying climate condensation in accordance with DIN EN ISO 6270-2 and spray mist test in accordance with DIN EN ISO 9227)

4.4. Corrosion protection systems

The resistance of corrosion protection systems in the form of organic coatings (such as powder coatings) must be determined depending on the required protection duration and the ambient conditions defined in DIN EN ISO 12944-2.

The duration of protection and the time periods for the duration of protection are defined in DIN EN ISO 12944-5. The duration of protection of powder coating systems depends on various parameters:

  • Design of the component and object
  • Stress after coating (location, use)
  • Condition of the steel surface or the zinc coating prior to preparation or pretreatment
  • Cleaning care and effectiveness of the pretreatment
  • Type of coating system The crucial factors here are:
    a) Number of coats (single or two-coat system)
    b) Thickness of the coats
    c) Penetration resistance of the coating system to corrosive atmospheres

DIN 55633, < Paints and varnishes – Corrosion protection of steel structures by powder coating systems – Assessment of powder coating systems and execution of coating > defined for the two basic substrate categories – unalloyed and low-alloyed steel – and for hot-dip galvanized steel defines recognized protection durations according to the classification in accordance with DIN EN ISO 12944-5.
Tables A.1 and A.2 in Appendix A specify the five relevant corrosion categories (C2, C3, C4, C5-I, and C5-M) and the coating thicknesses necessary for the protection durations (e.g., number of coats and coating thicknesses).

IGP Pulvertechnik AG meets these requirements with a series of anti-corrosive primers formulated to match various substrates and top coats. For further information, you can consult IGP documents and brochures: < Secure, lasting corrosion protection with IGP powder primers > and IGP corrosion protection tables for coating steel and aluminum substrates.

For in-depth information on anti-corrosion measures and systems, simply ask our qualified corrosion protection inspectors and expert consultants.