Hot Dip Aluminum Zinc Coatings

Many attempts have been made to improve the corrosion resistance of both galvanized and aluminum coatings through alloying. Although combinations of these two elements with each other were known to provide an attractive degree of corrosion resistance, hot dip coatings did not become feasible until the discovery that silicon inhibits the rapid alloying reaction with steel (Ref 26). The 55Al-Zn coating was first available commercially in 1972. This composition was selected from a systematic investigation of aluminum-zinc alloys, with up to 70% Al providing the best combination of galvanic protection and low corrosion rate. Current use varies from metal roofing, for which it is the major coated steel used, to automotive components, appliances, and, most recently, corrugated steel pipe.

Aluminum-zinc alloy coatings of steel sheet and wire are applied on continuous hot dip coating lines with in-line gas cleaning and heat treating of the steel substrate. With a nominal composition of 55% Al, 43.4% Zn, and 1.6% Si, the coating provides the durability and high temperature resistance of aluminum coatings with the sacrificial protection characteristics of zinc coatings. Silicon is added to the coating bath to control growth of an intermetallic layer. Steel sheet and wire products coated with 55% aluminum-zinc alloy are especially useful in applications requiring superior atmospheric corrosion resistance, cut edge protection, and/or high-temperature oxidation resistance. Processing details for aluminum-zinc coatings are given in Ref 16.

Microstructure of 55Al-Zn Coating. The 55Al-Zn coating has a two-phase structure of cored aluminum-rich dendrites and a zinc-rich interdendritic constituent (Fig. 12). This overlay is bonded to the steel substrate by a thin intermetallic layer whose composition is 48% Al, 24% Fe, 14% Zn, and 11% Si. X-ray diffraction suggests a structure similar to Al13Fe4. In addition, silicon particles are often found in the interdendritic region. By volume, the coating is approximately 80% Al + Si and 20% Zn. The effect of cooling rate during solidification is manifested in the spacing between the dendritic arms. Faster cooling (used commercially) results in finer spacing, which improves corrosion resistance.

Fig. 12 Microstructure of an aluminum-zinc coated sheet

Protection by Aluminum-Zinc Alloy Coatings. The 55Al-Zn coating provides both barrier and galvanic protection. Because the zinc-rich constituent is intimately distributed throughout the coating, it will be in contact with exposed steel at any break in the coating and at cut edges. Although less galvanic protection is available than with pure galvanized coatings, the alloy coating lasts longer because the overall corrosion rate is controlled by the aluminum-rich phase, which corrodes much more slowly than zinc.

Atmospheric Corrosion Resistance. Samples of 55Al-Zn-coated steel have been tested in atmospheric exposure for over 20 years. Figure 13 shows thickness loss with time for the first 13 years of exposure in four different atmospheres. Compared to galvanized panels exposed at the same time, the 55Al-Zn coating provides two to six times the corrosion resistance (based on equal coating thicknesses). Although these results were based on pilot line samples, subsequent testing of commercial 55Al-Zn sheet steel for 10 years shows a slightly greater advantage over galvanized steel (Ref 28).

Exposure time, years

Fig. 13 Coating thickness loss of 55Al-Zn-coated steel in four atmospheres. Source: Ref 27

Exposure time, years

Fig. 13 Coating thickness loss of 55Al-Zn-coated steel in four atmospheres. Source: Ref 27

Corrosion Mechanism. The zinc-rich constituent of the coating has been observed to corrode preferentially. As these regions are removed, their space is taken by corrosion products that become mechanically locked into the interdendritic spaces. These corrosion products are mostly amorphous aluminum or hydrated aluminum-zinc sulfates--similar to the corrosion products found on the coating surface of aluminum and 55Al-Zn coatings. These sulfates are adherent and may help explain the improved durability of the aluminum-zinc coating. Support for this mechanism is also obtained from aqueous corrosion studies in which the corrosion potential is observed to change that of a galvanized coating upon immersion to a value approaching that of aluminum after subsequent corrosion (Ref 29).

Aqueous Corrosion Resistance. The 55Al-Zn coating is finding increased use in applications demanding resistance to aqueous corrosion, especially where wet/dry cycles are obtained.

Corrosion in Natural Waters. As with other coatings, the corrosion of 55Al-Zn coating would be expected to vary with the specific properties of the water. It is not known how water hardness will affect corrosion, but in distilled water (very soft) and distilled water containing 85 mg/L of Cl- ion, the 55Al-Zn coating is much more resistant than a galvanized coating (Table 23). In similar tests, 55Al-Zn and galvanized panels were immersed for 90 days in distilled water containing 45 ppm of SO42- and 10 ppm of Cl- at pH values from 3 to 11 (Ref 31). The pH was maintained through sulfuric acid (H2SO4) or sodium hydroxide (NaOH) additions. As Table 24 demonstrates, the 55Al-Zn retains more coating than the galvanized at all pH values, especially within the 5 to 9 range most characteristic of natural waters.

Table 23 Average coating thickness loss of galvanized and 55Al-Zn-coated steel after 56 days of immersion

Material

Thickness loss

Distilled water

85 mg/L NaCl

fim

mils

^m

mils

Galvanized

1.06

0.042

1.26

0.049

55Al-Zn

0.015

0.0005

0.133

Table 24 Coating weight losses of galvanized and 55Al-Zn-coated steels after a 90-day immersion in water of various pHs

pH

Coating weight loss, %

Galvanized

55Al-Zn

3

32

31

5

45

2

7

41

4

9

40

8

11

33

26

The longest service history of exposure to natural water for the 55Al-Zn coating is obtained from corrugated steel pipe installed between October, 1973, and October, 1974 (Ref 32). Water chemistry, pH, and resistivity varied widely from site to site and sometimes changed considerably with time. Erosion and abrasion caused additional wear factors.

Overall, 55Al-Zn applied at a coating weight of 180 g/m2 (0.6 oz/ft2) provides greater durability than a 600 g/m2 (2 oz/ft2) galvanized coating. Continued monitoring of these sites suggests an average of 10 years of additional life for the 55Al-Zn coating in the pipe inverts, the point of severest corrosion and wear.

Corrosion in Soils. There are few performance data for the 55Al-Zn coating in soil. The corrugated steel pipe exposures described previously provide the longest history of soil exposure, but because most culverts fail from the inside, exterior soil corrosion was not monitored closely in these tests. Some data are available from laboratory tests in which 16-gage coated steel panels were buried in test soils and monitored for coating loss (Ref 32). Figure 14 shows coating loss and the soil characteristics. These data suggest that the 55Al-Zn coating should provide corrosion resistance in soil similar to that of a galvanized coating.

Soil number

Description

PH

Resistivity, fi' cm

1

Native shale, clay: wet to dry

4

76,000

2

Native shale with chloride and sulfate salts: wet and dry

5

35,000

3

Native shale, clay, and bentonite with chloride and sulfate salts: wet

6

1,700

Fig. 14 Corrosion of galvanized steel and 55Al-Zn-coated steel in three soils. Soil characteristics are also given.

Source: Ref 32

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