Chromate Conversion Coatings

Chromate conversion coatings are formed by a chemical or an electrochemical treatment of metals or metallic coatings in solutions containing hexavalent chromium (Cr6+) and, usually, other components. The process results in the formation of an amorphous protective coating composed of the substrate, complex chromium compounds, and other components of the processing bath.

Chromate conversion coatings are applied primarily to enhance bare or painted corrosion resistance, to improve the adhesion of paint or other organic finishes, and to provide the metallic surface with a decorative finish. Chromate conversion coatings are most frequently applied by immersion or spraying, but other methods of application, such as brushing, roll coating, dip and squeegee, electrostatic spraying, or anodic deposition, are used in special cases.

Processing Sequence. Chromate coatings are applied by contacting the processed surfaces with a sequence of processing solutions. The processing baths are arranged in a series of tanks, and the surfaces to be processed are transferred through the sequence of stages by using manual, semiautomatic, or automatic control. The chromate coatings are usually applied to metal parts or to a continuous metal strip running at speeds to 5 m/s (1000 ft/min).

The basic processing sequence consists of the following six steps: cleaning, rinsing, conversion coating, rinsing, post-treatment rinsing or decorative color rinsing, and drying. In many applications, this sequence is expanded to accommodate pickling deoxidizing, dyeing, brightening, and other rinsing stages, or the sequence can be shortened when cleaning or post-treatment rinsing is not necessary. Some typical processing sequences are given in Table 14.

Table 14 Typical process sequences for chromating of continuous galvanized strip

Step

Electrogalvanized strip

Step

Hot-dip galvanized strip

1

Electrogalvanizing

1

Hot-dip galvanizing

2

Rinsing: warm water, multiple stages, neutralizing (optional)

2

Surface conditioning: heat treatments

3

Conversion coating: chromate

3

Conversion coating: chromate

4

Rinsing: cold or warm water

4

Drying: warm air

5

Post-treatment: chromate (optional)

5

Oiling (optional)

6

Drying: warm air

7

Oiling or painting (optional)

Chromating Galvanized Steels. Although zinc-coated steels provide better atmospheric-corrosion resistance than bare cold-rolled steel, the natural resistance of zinc surfaces to atmospheric corrosion remains low. Most articles coated with zinc by electroplating or hot-dipping are further protected. Chromating is widely used to provide corrosion protection and a decorative finish in bare applications; it can also be used as a pretreatment before painting. The chromate coatings are formed on zinc surfaces from acid solutions containing Cr6+, usually other mineral acids, and accelerators. Chromate coatings on zinc can be categorized as clear, iridescent, and colored.

The largest area for the application of clear coatings is the passivation of hot-dip galvanized steel. Most unpainted zinc and zinc-aluminum coated steel sheet products used for roofs and walls of industrial and farm buildings are chromated on hot-dip galvanizing lines. These chemical treatments are applied in the coating weight range of 0.01 to 0.02 g Cr/m2 (1 to 2 mg Cr/ft2), with the upper limit usually defined by the color acceptable to the coated sheet users. On the strip line, coatings are applied by immersing or spraying the freshly prepared galvanized strip in chromating solutions for 1 to 3 s immediately before a set of squeegee rolls, which remove the excess solution from the strip. Broad ranges of chromium concentration and temperatures of 40 to 70 °C (100 to 160 °F) are used to achieve the desired coating weights. The corrosion protection provided by the chromate coatings increases with their color. Typical salt spray corrosion data are given in Table 15.

Table 15 Typical salt spray data for chromate coatings on zinc-coated steel

Substrate

Type of chromate coating

Time to corrosion stain, h

Electroplated zinc

Untreated

<4

Clear

24-48

Iridescent

100-200

Olive drab

100-400

Electrolytic

1000

Hot-dip zinc

Untreated

<4

Chromate coatings can be deposited electrolytically from baths containing CrO42 , PO43 , F-, and other anions. The resulting coatings, which are gray to black in appearance, have superior corrosion resistance and hardness when compared to the conversion coatings (Table 15).

Chromating Bare Steel. Both chemical and electrochemical methods for chromating steel have been used. The chemical method requires high temperatures and prolonged contact times. No-rinse chromate treatments are used on continuous strip lines for paint base applications. They are applied using chemical compositions and wet film thicknesses to give coating weights in the 0.16 to 0.38 g/m2 (15 to 35 mg/ft2) range and provide excellent paint base properties. Electrolytically produced chromium/chromium oxide coatings, better known as tin-free steel, are extensively used in the metal-packaging industry.

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