Effects of Anodic Coatings on Surface and Mechanical Properties

As the thickness of an anodic coating increases, light reflectance, both total and specular, decreases. This decrease is only slight for pure aluminum surfaces, but it becomes more pronounced as the content of alloying elements other than magnesium, which has little effect, increases. The decrease in reflectance values is not strictly linear with increasing thickness of anodic coating; the decrease in total reflectance levels off when the thickness of the coating on super-purity and high-purity aluminum is greater than about 2.5 pm (0.1 mil).

Data comparing the reflectance values of chemically brightened and anodized aluminum materials with those of other decorative materials are given in "Anodic Oxidation of Aluminium and Its Alloys," Bulletin 14 of the Aluminium Development Association (now the Aluminium Federation), London, England, 1949.

Table 8 shows the effect of anodized coatings 2 to 20 pm (0.08 to 0.8 mil) thick on the reflectance values of electrobrightened aluminum of three degrees of purity. This table also includes specular reflectance values for surfaces after removal of the anodic coating. These data show that the degree of roughening by the anodizing treatment increases as the purity of the aluminum decreases. The reflectance values of the anodized surfaces are influenced by the inclusion of foreign constituents or their oxides in the anodic coating.

Table 8 Effect of anodizing on reflectance values of electrobrightened aluminum

Thickness of anodic coating

Specular reflectance, %

Total reflectance after anodizing, %

m

mil

Electrobrightened

Electrobrightened and anodized

After removal, of anodic coating(a)

Aluminum, 99.99%

2

0.08

90

87

88

90

5

0.2

90

87

88

90

10

0.4

90

86

88

89

15

0.6

90

85

88

88

20

0.8

90

84

88

88

Aluminum, 99.8%

2

0.08

88

68

83

89

5

0.2

88

63

85

88

10

0.4

88

58

85

87

15

0.6

88

53

85

86

20

0.8

88

57

85

84

Aluminum, 99.5%

2

0.08

75

50

70

86

5

0.2

75

36

64

84

10

0.4

75

26

61

81

15

0.6

75

21

57

77

20

0.8

75

15

53

73

Source: Aluminum Development Council

Source: Aluminum Development Council

(a) Anodic coating removed in chromic-phosphoric acid.

Metallurgical factors have a significant influence on the effect of anodizing on reflectance. For minimum reduction in reflectance, the conversion of metal to oxide must be uniform in depth and composition. Particles of different composition do not react uniformly. They produce a nonuniform anodic coating and roughen the interface between the metal and the oxide coating.

Anodizing Conditions. The composition and operating conditions of the anodizing electrolyte also influence the light reflectance and other properties of the polished surface. Figure 8 shows the effect of sulfuric acid concentration, temperature of bath, and current density on the specular reflectance of chemically brightened aluminum alloy 5457. These data show that a particular level of specular reflectance can be produced by varying operating conditions.

Fig. 8 Effect of anodizing conditions on specular reflectance of chemically brightened aluminum. Data are for a 5 pm (0.2 mil) anodic coating on 5457 alloy. (a) 17 wt% H2SO4. (b) 8.8 wt% H2SO4

Thermal Radiation. The reflectance of aluminum for infrared radiation also decreases with increasing thickness of the anodic coating, as shown in Fig. 9. These data indicate that the difference in purity of the aluminum is of minor significance. Figure 10 compares anodized aluminum surfaces and polished aluminum surfaces at 21 °C (70 °F) with respect to absorptance when exposed to blackbody radiation from sources of different temperatures. Although anodized aluminum is a better absorber of low-temperature radiation, as-polished aluminum is a more effective absorber of blackbody radiation from sources at temperatures exceeding 3300 °R (1850 K).

Thickness of anodic film, mils 0.1 0.2 0-3 0.4 0-5 0,6 0.7

■ ^

—1—

r

l

T

O

Thick new of anodic li(mr jim

Thick new of anodic li(mr jim

Fig. 9 Effect of anodic coating thickness on reflectance of infrared radiation. Temperature of infrared radiation source, 900 °C (1650 °F). O: 99.99% AI. •: 99.50% Al. Courtesy of Aluminum Development Council

Fig. 9 Effect of anodic coating thickness on reflectance of infrared radiation. Temperature of infrared radiation source, 900 °C (1650 °F). O: 99.99% AI. •: 99.50% Al. Courtesy of Aluminum Development Council

Fig. 10 Comparison of absorptance of blackbody radiation by anodized aluminum and polished aluminum. Temperature of aluminum surface. 530 °R (21 °C, or 70 °F)

Fatigue Strength. Anodic coatings are hard and brittle, and they will crack readily under mechanical deformation. This is true for thin as well as thick coatings, even though cracks in thin coatings may be less easily visible. Cracks that develop in the coating act as stress raisers and are potential sources of fatigue failure of the substrate metal. Typical fatigue-strength values for aluminum alloys before and after application of a hard anodic coating 50 to 100 pm (2 to 4 mils) thick are given in Table 9.

Table 9 Effect of anodizing on fatigue strength of aluminum alloys

Not anodized

Anodized

MPa

ksi

MPa

ksi

Wrought alloys

2024 (bare)

130

19

105

15

2024 (clad)

75

11

50

7.5

6061 (bare)

105

15

40

6

7075 (bare)

150

22

60

9

7075 (clad)

85

12

70

10

Casting alloys

220

50

7.5

52

7.5

356

55

8

55

8

Note: Values are for sulfuric acid hard coatings 50 to 100 pm (2 to 4 mils) thick applied using 15% sulfuric acid solution at -4 to 0 °C (25 to 32 °F) and 10 to 75 V dc.

Source: F.J. Gillig, WADC Technical Report 53-151, P.B. 111320, 1953

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