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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.