- Email: [email protected]
Production and characterization of the anodic film on Al-6%Zn-1%Mg alloy
Surface and Coatings Technology, 34 (1988) 231
-
241
231
PRODUCTION AND CHARACTERIZATION OF THE ANODIC FILM ON A1-6%Zn--1%Mg ALLOY R. J. TZOU and H. C. SHIH Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043 (Taiwan) (Received January 16, 1987)
Summary The newly developed high strength aluminum alloy Al—6wt.%Zn-lwt.%Mg was anodized in acids by direct current to produce a protective anodic coating against abrasion and corrosion. Mechanical properties of the anodized finish, including microhardness, abrasion resistance and fatigue strength were evaluated in terms of the composition and temperature of the electrolyte, applied voltage, current density, duration of anodizing and sealing temperature. Pitting is destructive of the anodized finish particularly in a marine environment and thus can also serve as a direct measure of the efficacy of sealing procedures. The optimum conditions for anodizing Al—6wt.%Zn~1wt.%Mg are a bath of 5 wt.% oxalic acid with 3 wt.% formic acid as an additive, a current density of 4 A dm2 for 30 mm at 20 °Cand sealing in boiling deionized water for 30 mm. Results show that this anodized finish possesses good mechanical properties and corrosion resistance.
1. Introduction A1—6wt.%Zn—lwt.%Mg alloy (A16Zn1Mg) possesses superior mechanical properties such as an ultimate tensile strength of 35 50 kg mm2, a yield strength of 25 45 kg mm2 and an elongation of 12% after appropriate thermomechanical treatments [1, 2]. Although high strength aluminum alloys achieve these good mechanical properties as a result of precipitation, often the atmospheric corrosion resistance is significantly reduced, particularly in the climate of semitropical islands such as Taiwan. These atmospheres, laden with hygroscopic impurities and moisture, establish a conducive environment for corrosion. A16Zn1Mg has been found to be particularly susceptible to pitting corrosion compared with other aluminum alloys [31 since the natural oxide coating is usually not dense enough to protect the alloy from the electrochemical action of chloride penetration. However, some success has -
-
0257-8972/88/$3.50
© Elsevier
Sequoia/Printed in The Netherlands
232
been achieved by applying corrosion inhibitors to a closed system, e.g. the addition of 0.5 wt.% NaNO3 to 3 wt.% NaCl solution has been reported to be very effective in the reduction of corrosion [4]. The major attack of aluminum alloys in sea water was found to be in the form of pitting and exfoliation [5]. Because of its strong affinity to oxygen, aluminum forms an oxide surface film. This film can be further thickened and strengthened by means of an anodizing treatment in an appropriate electrolyte to achieve the required properties including microhardness, abrasion and corrosion resistances and fatigue strength. The physical properties of the anodized film normally depend on the composition of the bath and the production conditions including temperature, applied voltage, current density, duration of anodizing and sealing temperature. The main advantage of the oxalic acid process today is that the colors obtainable are sufficiently attractive to avoid the need for dyeing. Colors may range from silver to bronze depending on the base metal and coating thickness. This process is more expensive than the sulfuric acid process both in chemicals and current, but thicker coatings of up to 60 jim may be obtained without the use of special techniques. Furthermore, porous and thick anodic coatings were also formed in formic acid at a relatively high temperature and concentration [6]. Although the addition of formic acid to oxalic acid has been developed as a technique for the rapid formation of thick anodic films on aluminum and its alloys, the characterization of this coating required investigation. Sulfuric acid, oxalic acid and chromic acid have been well documented as the major baths for anodizing aluminum. However, the wrought Al—Zn— Mg alloys are a little more difficult to coat and cooling is more important. A successful anodizing treatment is limited to only a few per cent of alloying elements in aluminum alloys which derive their improved strength from the precipitation of intermetallic compounds along slip planes and grain boundaries. This process results in a depletion of the alloying element in the alloy adjacent to the intermetallic compound, accounting for grain boundaries more anodic to the grains and a marked susceptibility to localized corrosion. A porous structure superimposed on a compact layer normally occurs in solutions in which the oxide has a moderate solubility. If the solubility is very low, such as in boric acid, only a compact thin oxide is produced. Two models, known as the KHR model [7] and colloidal gel model [81, are usually applied to describe the mechanism of aluminum oxide formation. Maeland et at. [9] used JR spectroscopy to determine the composition and structure of the barrier layer and found that this peculiar layer is composed of dehydrated aluminum oxide. This finding is consistent with the KHR model. Many techniques have been proposed to study the growth mechanism of porous anodic films on aluminum [10 15]. The pores can be closed by exposure to steam or hot water, a process of chemical transformation -
233
which must be applied after anodic oxidation. Sealed anodized aluminum oxides provide very good corrosion resistance. Various methods are available for the sealing process depending on its purpose, and in this work only hot water was used as the sealing medium. Electrochemical polarization measurements are sensitive to the presence of surface film and could possibly lead to improved methods for evaluating anodizing. In this investigation, anodized coatings of varying thickness and degree of seal were compared by potentiodynamic anodic polarization.
2. Experimental details AI6Zn1Mg alloys were melted in a graphite crucible and the castings were homogenized at 470 °Cfor 24 h and hot rolled at 400 °Cto a thickness of 1 mm. The chemical composition was analyzed by a spark emission spectrometer to give the results shown in Table 1. TABLE 1 Chemical composition of A16Zn1Mg alloy Element Content (wt.%)
Zn 5.6
Mg 0.81
Mn 0.21
Zr 0.21
Cr 0.01
Fe 0.01
Cu 0.04
Al Remainder
Specimens for anodizing were 7 cm long by 2 cm wide, and a 1.5 mm diameter hole was drilled near the edge of each sample for mounting. The final heat treatment was performed at 413 °Cfor 2.5 h and was followed by air cooling. Each face was given a standard 600 grit SiC finish. All specimens were degreased ultrasonically in trichloroethylene and rinsed in distilled water. Aluminum wires were thereafter used for connecting the coupon to the power supply. In order to achieve a uniform and film-free surface, all test coupons were electropolished in an electrolyte bath under the following conditions: electrolyte, 60% perchloric acid and absolute ethanol (1:4 v/v); current density, 0.3 A cm~2 voltage, 12 V; temperature, 10 °C;time, 3 mm. Prior to anodizing, the coupons were rinsed with methanol, dried in cold air, and stored in a desiccator ready for use. 2.1. Anodizing The circuit diagram for anodizing A16Zn1Mg comprised a d.c. power supply (Philips PE1511), an ammeter (Fluke 800A digital multimeter) in series and a voltage recorder (HP 7155A) in parallel. A thermostat (Forma Scientific CH/P 2067) with a control sensitivity of ±0.02°Cmaintained a constant temperature in the range —20 +80 °Cduring anodizing. Two lead plates were used as cathodes opposing the two surfaces of the specimen to -
234
produce a uniform current distribution. An air pump was also used to agitate the electrolyte to avoid local heating. The anodic oxidation of A16Zn1Mg was performed primarily in oxalic acid plus additives of formic acid and sulfuric acid. The following electrolytes were used: 5 wt.% oxalic acid (electrolyte 0500); 5 wt.% oxalic acid plus 3 wt.% formic acid (electrolyte 0503); 12 wt.% sulfuric acid (electrolyte S012). The test bath was prepared from analytical grade acids. The anodized coupons were thoroughly cleaned in deionized water and subsequently immersed in boiling deionized water (pH 6.1) for sealing. The anodic oxide thickness was checked non-destructively by the standard eddycurrent technique according to ASTM B244, and its microhardness, corrosion, abrasion and fatigue resistances, which are of relevance to its efficacy on engineering material, were measured accordingly.
2.2. Weight loss test Anodized specimens were immersed in the test solution (25 g 1_i NaC1, 15 ml 1_i acetic acid and 3.3 ml 1’ H202) according to DIN 50947 at room temperature for 6 days, washed in soapy water with a soft nylon brush, then ultrasonically cleaned in pure water and finally dried in hot air ready for weighing on an analytical balance accurate to within ±0.1mg. 2.3. Pitting resistance Polarization was carried out in a cell and with the electrical circuitry described elsewhere [161. The cell consisted of a multineck round-bottom Pyrex flask incorporating the working electrode, auxiliary electrode, a Luggin probe with a solution bridge to a saturated calomel reference electrode (SCE) and a bubbler tube for aerating with an air pump. The pitting resistance of anodized A16Zn1Mg was evaluated in natural sea water by a Wenking potentiostat (P0S73). A scan rate of 60 mV min~’was found to be convenient to use and sufficiently slow to avoid any loss of configuration on the potentiodynamic polarization curves.
2.4. Abrasion resistance A Taber Abraser Model 503 was used for testing the wear characteristics of the anodic coating of A16Zn1Mg by weight loss measurements. The effect of the anodizing temperature on the abrasion resistance was measured.
2.5. Fatigue resistance Resistance to fatigue in the atmosphere was evaluated in terms of the plots of stress vs. number of cycles (S—N curves) for Al6ZnlMg subject to cyclic stresses (Allen—Bradley).
235
3. Results and discussion Figures 1 and 2 show the effect of the anodizing temperature on the film thickness and the microhardness respectively. It is apparent that baths containing oxalic acid produce a greater film thickness. Owing to the anodizing mechanism of dissolution—deposition, the deposition efficiency is higher in oxalic acid than in sulfuric acid. This may be ascribed to the stronger adsorbing or chelating ability of oxalate anions than sulfate anions for the hydrated Al3~ions [10]. Furthermore, it appears that the addition of 3% formic acid to 5% oxalic acid (electrolyte 0503) increases the microhardness of AI6Zn1Mg substantially. This is unequivocally true when the anodic current density reaches or exceeds 4 A dm2 where a microhardness as high as 660 kg mm2 can be obtained, as shown in Fig. 3. These results also indicate that the higher the anodizing temperature the thinner the anodic oxide thickness. This behavior is simply due to the higher dissolution rate of aluminum oxide at higher temperatures in this particular bath. The color appearance of the anodic film by visual inspection is influenced by the film thickness or current density, i.e. the thinner film shows a silver or transparent color, whereas a yellow—bronze color is apparent for the thicker coating obtained from the oxalic and/or additive acid bath. However, the film color is always a semitransparent white in the sulfuric acid electrolyte. Before the sealing treatment, the anodized film exhibits a higher microhardness. Sealing at 75 °Cfor 60 mm produces a lower microhardness as I
I 700
30 60025
15
0503
~osco
~0503
30
Anodizing temperature (C)
300
S012
AriodizirIg
2~
ternp.roture C C I
Fig. 1. Effect of anodizing temperature on film thickness at 3 A dm2 for 30 (coatings sealed in boiling water for 30 mm).
mm
Fig. 2. Effect of anodizing temperature on microhardness at 3 A dm2 for 30 (coatings sealed in boiling water for 30 mm).
mm
~
236
2
Current density (A/drn
Fig. 3. Effect of current density on microhardness (at 20 °Cfor 30 mm; coatings sealed in boiling water for 30 mm).
~
60G
~
4cc
I
~1 (I, In
I
I
.5 0 25
I
I
50
75
I
100 Sealing T~npernture (C)
125
Fig. 4. Effect of sealing temperature on microhardness (electrolyte S012 with 3 A dm2 at 20 °Cfor 30 mm).
shown in Fig. 4. The lower microhardness is apparently accounted for by the formation of Bayerite, Al 2033H20. Furthermore, sealing at boiling temperature favors the formation of Boehmite, Al203H20 [15], which is harder than A12O33H2O but softer than unhydrated A1203. Figure 4 indicates that anodized coatings after various sealing temperatures have about 70% 80% of the microhardness of the coating sealed at 25 °C. The weight loss on sealing at 25 °Ccould be 13-fold that on sealing at 100 °Cas shown in Fig. 5(a). It is apparent that the deterioration or failure, as shown in Fig. 5(b), in the accelerating corrosion test occurs most readily with anodized specimens which are unsealed or poorly sealed. Therefore, it is now axiomatic that sealing of the anodic coating is the most important requirement. The effect of the anodizing temperature on the abrasion resistance is shown in Fig. 6. It is readily apparent that the lower anodizing temperature, such as 5 °C, produces more resistance to abrasion and consequently less weight loss of the anodized coating. This may be related to the fact that the residual stress of the anodized film formed at 5 Dc is higher than that formed at 20 °C. -
237
25
50
75
~J0
401-’-
125
Sealing Temperature VC)
I
(a)
_J
(h)
Fig. 5. (a) Effect on sealing temperature on weight loss (electrolyte S012 with 3 A d~n 2 at 20 °C for 30 mm; coatings sealed for 30 mm). (b) Film deterioration due to r~oorly sealed coating (magnification x 264). I
I
120
I
I
I
-
Cycles
(elOO)
Fig. 6. Abrasion resistance of anodized Al6ZnlMg (abraser load, 250 gf) in terms of 2 for 30 mm). weight loss (electrolyte 0503 with 4 A dm
Electrochemical polarizations were applied to evaluate the pitting resistance of Al6ZnlMg in fresh sea water. The chemical composition of sea water is shown in Table 2. The effect of sealing time on the passive current density is shown in Fig. 7. The anodic polarization curve for an unanodized specimen is also shown for comparison. Each specimen was anodized in bath 0503 with 4 A dm~2at 20 °Cfor 30 mm prior to sealing to produce a 25 jim coating. It is obvious that the effect of sealing time on the anodic behavior of this alloy is important, i.e. the longer the sealing time, the more pitting resistant the oxide film. A16Zn1Mg with zero sealing time possesses a discernible range of passivity which breaks down by pitting at —0.5 V(SCE) as shown in Fig. 7. Furthermore, sealing for 10 mm can effectively suppress the rapid pitting of the unsealed coating. The extent of anodic polarization
238 TABLE 2 The chemical composition of natural sea watera 2~
Ion species
Na~
CU
Mg
Concentration (M)
0.456
0.5339
0.0537
SO~2
K~
Ca2~
HC0
0.0272
0.0098
0.0693
0.0023
3
apH 8.2; specific gravity, 1.026.
0~IIIII~II~
IIIIII~II
I’~l~l
II~~I I
2 Current derluity ( A/cm Fig. 7. Effect of sealing time on the anodic behavior of anodized Al6ZnlMg (electrolyte 0503 with 4 A dm~2at 20 °Cfor 30 mm).
is apparently quite sensitive to the degree of sealing. Sealing for 20 and 30 mm further lowers the passive current densities. Figure 8(a) shows the effect of anodizing time on the anodic behavior of Al6ZnlMg in the same bath (0503) at 20 °Cand 4 A dm2. Each anodized coupon was sealed in boiling deionized water at 100 °Cfor 20 mm before its anodic polarization in aerated sea water. A similar decay of passive current density was also observed as the time for anodizing increased, as shown in Fig. 8(b). It is readily seen that a longer anodizing time as well as sealing time decreases the passive current densities. The anodized coating on aluminum is generally considered to have a duplex structure, i.e. a dense compact barrier layer next to the metal is surmounted by an outer porous layer. The porous layer has long been considered as consisting of oxide penetrated in a regular manner by columnar pores extending from the outer surface to the porous-barrier interface. However, in this context, the barrier layer itself provides little resistance to chloride penetration during anodic polarization, since the unsealed coatings were readily pitted. This result agrees with the previous mechanism of anodizing. Furthermore, the sealed oxide structure provides a strong resistance to pitting and anodic current flow. Therefore, anodic polarization is a good indicator and prediction of the resistance of the coating to breakdown by
239
JmIn8mIn5mI~,,,//)3mIn
2
(a)
Current density CA/cm .3
(b)
I
_____________________________________________________
Anodizing Time (mill.
Fig. 8. (a)Effectofanodizingtimeonthe anodic behavior of Al6AnlMg (electrolyte 0503 at 20 °C and 4 A dm2). For anodizing times of 10 mm, 8 mm, 5 mm and 3 mm, the film thickness was 9 pm, 7 pm, 4 pm and 2 pm respectively. (b) Effect of anodizing time on the passive current density of (a) at ÷1.6 V (SCE).
natural sea water. Jones [17] also demonstrated that the anodic polarization behavior of anodized coatings parallels their performance in atmospheric service and could be used as a test for the atmospheric corrosion resistance of anodized coatings. Pitting corrosion, as shown in Fig. 9, is confined to weak sites in the anodic coating which are usually of random distribution and not clearly connected with known metallurgical features, although the form of the intermetallic constituent is obviously connected with them because pitting decreases as the purity of the metal is increased. This is frequently the mode of deterioration of a well-sealed coating, the frequency of pitting being an inverse function of the thickness of the coating. It is also typical of marine environments which in themselves are not particularly hostile towards the
240
aluminum oxide film. Industrial conditions can of course be selectively aggressive in the same way. The fatigue resistance of A16Zn1Mg related to the different surface conditions is outlined in Table 3. These results are also presented in Fig. 10 in which heat treatment 4, i.e. the aged and anodized A16Zn1Mg, possesses the highest resistance to fatigue compared with the other surface treatments investigated. TABLE 3 Fatigue resistance of Al6ZnlMg after various treatments Heat treatment
Anodizing
Cycles before fracture at 40 ksi~
1. Annealed at 413 °Cfor 2.5 h and air cooled 2. Aged at 135 °C for iO~mm 3. Annealed at 413 °C for 2.5 h and air cooled 4. Aged at 135 °C for iO~mm
None
3.5
X
iO~
None
4.6
x
iO~
In 0503 at 20 °C, 2for 4 A dm~ 30 mm In 0503 at 20 °C, 4 A dm2 for 30 mm
6.0
x
8.0
x
*
Kilo pound per square inch.
~s0~_
Number of Cycles
Fig. 9. SEM photograph of the unsealed coating on Al6ZnlMg anodized in electrolyte 0503 with 4 A dm2 at 20 °Cfor 30 mm (magnification x273). Fig. 10. Plots of stress vs. number of cycles (S—N curves) for Al6AnlMg subjected to cyclic stress: 0, fully annealed; X, aged at 135 °Cfor iO~mm; A, anodized; •, aged and anodized.
241
4. Conclusion In summary, it can be stated that the resistance to general corrosion as well as pitting corrosion of A16Zn1Mg is significantly improved after anodizing in an appropriate bath. Anodizing at lower temperatures was found to give better resistance to wear. Judging from the limited data obtained so far, it appears that anodized AI6Zn1Mg possesses a better resistance to fatigue than the untreated alloy. The optimum anodizing conditions for this particular alloy as found in this study are a 5% oxalic acid plus 3% formic acid (electrolyte 0503) bath with a current density of 4 A dm2 for 30 mm at 20 °C.The resulting microhardness is 660 kg mm2 which is equivalent to that of hard chromium plating and the Taber Abraser Index appraoches 75.
Acknowledgment The authors wish to thank Professor K. S. Liu of the National Tsing Hua University for providing the Al—6%Zn—1%Mg alloy and for helpful discussions.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K. S. Liu, J. L. Horng and T. B. Wu, Chin. J. Mater. Sci., 11(1) (1979) 63. J. W. Yeh and K. S. Liu, Scr. Metall., 20 (3) (1986) 329. H. C. Shih and J. Y. Wu, Chin. J. Mater. Sci., 11(4) (1979) 85. C. H. Yu, MS. Thesis, National Tsing Hua University, 1979. E. Mattsson, L. 0. Gullman, L. Knutsson, R. Sundberg and B. Thundal, Br. Corros. J., 6 (1971) 73. 5. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and Its Alloys, Draper, Teddington, 4th edn., 1 (1972) 454. F. Keller, M. S. Hunter and D. L. Robinson, J. Electrochem. Soc., 100 (1953) 411. C. E. Michelson, J. Electrochem. Soc., 115 (1968) 213. A. J. Maeland, R. C. Rittenhouse and K. Bird, J. Plating Surf. Finishing, 63 (1976) 56. G. C. Tu, Chin. J. Mater. Sci., 18 (2) (1986) 90. K. Wefers and W. T. Evans, J. Plating Surf. Finishing, 62 (1975) 951. G. C. Wood, J. P. 0’SullivanandB.Vaszko,J. Electrochem. Soc., 115 (6) (1968)618. W. J. Bernard and J. W. Cook, J. Electrochem. Soc., 106 (8) (1959) 643. J. Zahavi and M. Metzger, J. Electrochem. Soc., 119 (11) (1972) 1479. R. K. Hart, Trans. Faraday Soc., 50 (1954) 269. N. D. Greene, Experimental Electrode Kinetics, Rensselaer Polytechnic Institute, Troy, NY, 1965. D. A. Jones, Corrosion (Houston), 25 (4) (1969) 187.
-
241
231
PRODUCTION AND CHARACTERIZATION OF THE ANODIC FILM ON A1-6%Zn--1%Mg ALLOY R. J. TZOU and H. C. SHIH Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043 (Taiwan) (Received January 16, 1987)
Summary The newly developed high strength aluminum alloy Al—6wt.%Zn-lwt.%Mg was anodized in acids by direct current to produce a protective anodic coating against abrasion and corrosion. Mechanical properties of the anodized finish, including microhardness, abrasion resistance and fatigue strength were evaluated in terms of the composition and temperature of the electrolyte, applied voltage, current density, duration of anodizing and sealing temperature. Pitting is destructive of the anodized finish particularly in a marine environment and thus can also serve as a direct measure of the efficacy of sealing procedures. The optimum conditions for anodizing Al—6wt.%Zn~1wt.%Mg are a bath of 5 wt.% oxalic acid with 3 wt.% formic acid as an additive, a current density of 4 A dm2 for 30 mm at 20 °Cand sealing in boiling deionized water for 30 mm. Results show that this anodized finish possesses good mechanical properties and corrosion resistance.
1. Introduction A1—6wt.%Zn—lwt.%Mg alloy (A16Zn1Mg) possesses superior mechanical properties such as an ultimate tensile strength of 35 50 kg mm2, a yield strength of 25 45 kg mm2 and an elongation of 12% after appropriate thermomechanical treatments [1, 2]. Although high strength aluminum alloys achieve these good mechanical properties as a result of precipitation, often the atmospheric corrosion resistance is significantly reduced, particularly in the climate of semitropical islands such as Taiwan. These atmospheres, laden with hygroscopic impurities and moisture, establish a conducive environment for corrosion. A16Zn1Mg has been found to be particularly susceptible to pitting corrosion compared with other aluminum alloys [31 since the natural oxide coating is usually not dense enough to protect the alloy from the electrochemical action of chloride penetration. However, some success has -
-
0257-8972/88/$3.50
© Elsevier
Sequoia/Printed in The Netherlands
232
been achieved by applying corrosion inhibitors to a closed system, e.g. the addition of 0.5 wt.% NaNO3 to 3 wt.% NaCl solution has been reported to be very effective in the reduction of corrosion [4]. The major attack of aluminum alloys in sea water was found to be in the form of pitting and exfoliation [5]. Because of its strong affinity to oxygen, aluminum forms an oxide surface film. This film can be further thickened and strengthened by means of an anodizing treatment in an appropriate electrolyte to achieve the required properties including microhardness, abrasion and corrosion resistances and fatigue strength. The physical properties of the anodized film normally depend on the composition of the bath and the production conditions including temperature, applied voltage, current density, duration of anodizing and sealing temperature. The main advantage of the oxalic acid process today is that the colors obtainable are sufficiently attractive to avoid the need for dyeing. Colors may range from silver to bronze depending on the base metal and coating thickness. This process is more expensive than the sulfuric acid process both in chemicals and current, but thicker coatings of up to 60 jim may be obtained without the use of special techniques. Furthermore, porous and thick anodic coatings were also formed in formic acid at a relatively high temperature and concentration [6]. Although the addition of formic acid to oxalic acid has been developed as a technique for the rapid formation of thick anodic films on aluminum and its alloys, the characterization of this coating required investigation. Sulfuric acid, oxalic acid and chromic acid have been well documented as the major baths for anodizing aluminum. However, the wrought Al—Zn— Mg alloys are a little more difficult to coat and cooling is more important. A successful anodizing treatment is limited to only a few per cent of alloying elements in aluminum alloys which derive their improved strength from the precipitation of intermetallic compounds along slip planes and grain boundaries. This process results in a depletion of the alloying element in the alloy adjacent to the intermetallic compound, accounting for grain boundaries more anodic to the grains and a marked susceptibility to localized corrosion. A porous structure superimposed on a compact layer normally occurs in solutions in which the oxide has a moderate solubility. If the solubility is very low, such as in boric acid, only a compact thin oxide is produced. Two models, known as the KHR model [7] and colloidal gel model [81, are usually applied to describe the mechanism of aluminum oxide formation. Maeland et at. [9] used JR spectroscopy to determine the composition and structure of the barrier layer and found that this peculiar layer is composed of dehydrated aluminum oxide. This finding is consistent with the KHR model. Many techniques have been proposed to study the growth mechanism of porous anodic films on aluminum [10 15]. The pores can be closed by exposure to steam or hot water, a process of chemical transformation -
233
which must be applied after anodic oxidation. Sealed anodized aluminum oxides provide very good corrosion resistance. Various methods are available for the sealing process depending on its purpose, and in this work only hot water was used as the sealing medium. Electrochemical polarization measurements are sensitive to the presence of surface film and could possibly lead to improved methods for evaluating anodizing. In this investigation, anodized coatings of varying thickness and degree of seal were compared by potentiodynamic anodic polarization.
2. Experimental details AI6Zn1Mg alloys were melted in a graphite crucible and the castings were homogenized at 470 °Cfor 24 h and hot rolled at 400 °Cto a thickness of 1 mm. The chemical composition was analyzed by a spark emission spectrometer to give the results shown in Table 1. TABLE 1 Chemical composition of A16Zn1Mg alloy Element Content (wt.%)
Zn 5.6
Mg 0.81
Mn 0.21
Zr 0.21
Cr 0.01
Fe 0.01
Cu 0.04
Al Remainder
Specimens for anodizing were 7 cm long by 2 cm wide, and a 1.5 mm diameter hole was drilled near the edge of each sample for mounting. The final heat treatment was performed at 413 °Cfor 2.5 h and was followed by air cooling. Each face was given a standard 600 grit SiC finish. All specimens were degreased ultrasonically in trichloroethylene and rinsed in distilled water. Aluminum wires were thereafter used for connecting the coupon to the power supply. In order to achieve a uniform and film-free surface, all test coupons were electropolished in an electrolyte bath under the following conditions: electrolyte, 60% perchloric acid and absolute ethanol (1:4 v/v); current density, 0.3 A cm~2 voltage, 12 V; temperature, 10 °C;time, 3 mm. Prior to anodizing, the coupons were rinsed with methanol, dried in cold air, and stored in a desiccator ready for use. 2.1. Anodizing The circuit diagram for anodizing A16Zn1Mg comprised a d.c. power supply (Philips PE1511), an ammeter (Fluke 800A digital multimeter) in series and a voltage recorder (HP 7155A) in parallel. A thermostat (Forma Scientific CH/P 2067) with a control sensitivity of ±0.02°Cmaintained a constant temperature in the range —20 +80 °Cduring anodizing. Two lead plates were used as cathodes opposing the two surfaces of the specimen to -
234
produce a uniform current distribution. An air pump was also used to agitate the electrolyte to avoid local heating. The anodic oxidation of A16Zn1Mg was performed primarily in oxalic acid plus additives of formic acid and sulfuric acid. The following electrolytes were used: 5 wt.% oxalic acid (electrolyte 0500); 5 wt.% oxalic acid plus 3 wt.% formic acid (electrolyte 0503); 12 wt.% sulfuric acid (electrolyte S012). The test bath was prepared from analytical grade acids. The anodized coupons were thoroughly cleaned in deionized water and subsequently immersed in boiling deionized water (pH 6.1) for sealing. The anodic oxide thickness was checked non-destructively by the standard eddycurrent technique according to ASTM B244, and its microhardness, corrosion, abrasion and fatigue resistances, which are of relevance to its efficacy on engineering material, were measured accordingly.
2.2. Weight loss test Anodized specimens were immersed in the test solution (25 g 1_i NaC1, 15 ml 1_i acetic acid and 3.3 ml 1’ H202) according to DIN 50947 at room temperature for 6 days, washed in soapy water with a soft nylon brush, then ultrasonically cleaned in pure water and finally dried in hot air ready for weighing on an analytical balance accurate to within ±0.1mg. 2.3. Pitting resistance Polarization was carried out in a cell and with the electrical circuitry described elsewhere [161. The cell consisted of a multineck round-bottom Pyrex flask incorporating the working electrode, auxiliary electrode, a Luggin probe with a solution bridge to a saturated calomel reference electrode (SCE) and a bubbler tube for aerating with an air pump. The pitting resistance of anodized A16Zn1Mg was evaluated in natural sea water by a Wenking potentiostat (P0S73). A scan rate of 60 mV min~’was found to be convenient to use and sufficiently slow to avoid any loss of configuration on the potentiodynamic polarization curves.
2.4. Abrasion resistance A Taber Abraser Model 503 was used for testing the wear characteristics of the anodic coating of A16Zn1Mg by weight loss measurements. The effect of the anodizing temperature on the abrasion resistance was measured.
2.5. Fatigue resistance Resistance to fatigue in the atmosphere was evaluated in terms of the plots of stress vs. number of cycles (S—N curves) for Al6ZnlMg subject to cyclic stresses (Allen—Bradley).
235
3. Results and discussion Figures 1 and 2 show the effect of the anodizing temperature on the film thickness and the microhardness respectively. It is apparent that baths containing oxalic acid produce a greater film thickness. Owing to the anodizing mechanism of dissolution—deposition, the deposition efficiency is higher in oxalic acid than in sulfuric acid. This may be ascribed to the stronger adsorbing or chelating ability of oxalate anions than sulfate anions for the hydrated Al3~ions [10]. Furthermore, it appears that the addition of 3% formic acid to 5% oxalic acid (electrolyte 0503) increases the microhardness of AI6Zn1Mg substantially. This is unequivocally true when the anodic current density reaches or exceeds 4 A dm2 where a microhardness as high as 660 kg mm2 can be obtained, as shown in Fig. 3. These results also indicate that the higher the anodizing temperature the thinner the anodic oxide thickness. This behavior is simply due to the higher dissolution rate of aluminum oxide at higher temperatures in this particular bath. The color appearance of the anodic film by visual inspection is influenced by the film thickness or current density, i.e. the thinner film shows a silver or transparent color, whereas a yellow—bronze color is apparent for the thicker coating obtained from the oxalic and/or additive acid bath. However, the film color is always a semitransparent white in the sulfuric acid electrolyte. Before the sealing treatment, the anodized film exhibits a higher microhardness. Sealing at 75 °Cfor 60 mm produces a lower microhardness as I
I 700
30 60025
15
0503
~osco
~0503
30
Anodizing temperature (C)
300
S012
AriodizirIg
2~
ternp.roture C C I
Fig. 1. Effect of anodizing temperature on film thickness at 3 A dm2 for 30 (coatings sealed in boiling water for 30 mm).
mm
Fig. 2. Effect of anodizing temperature on microhardness at 3 A dm2 for 30 (coatings sealed in boiling water for 30 mm).
mm
~
236
2
Current density (A/drn
Fig. 3. Effect of current density on microhardness (at 20 °Cfor 30 mm; coatings sealed in boiling water for 30 mm).
~
60G
~
4cc
I
~1 (I, In
I
I
.5 0 25
I
I
50
75
I
100 Sealing T~npernture (C)
125
Fig. 4. Effect of sealing temperature on microhardness (electrolyte S012 with 3 A dm2 at 20 °Cfor 30 mm).
shown in Fig. 4. The lower microhardness is apparently accounted for by the formation of Bayerite, Al 2033H20. Furthermore, sealing at boiling temperature favors the formation of Boehmite, Al203H20 [15], which is harder than A12O33H2O but softer than unhydrated A1203. Figure 4 indicates that anodized coatings after various sealing temperatures have about 70% 80% of the microhardness of the coating sealed at 25 °C. The weight loss on sealing at 25 °Ccould be 13-fold that on sealing at 100 °Cas shown in Fig. 5(a). It is apparent that the deterioration or failure, as shown in Fig. 5(b), in the accelerating corrosion test occurs most readily with anodized specimens which are unsealed or poorly sealed. Therefore, it is now axiomatic that sealing of the anodic coating is the most important requirement. The effect of the anodizing temperature on the abrasion resistance is shown in Fig. 6. It is readily apparent that the lower anodizing temperature, such as 5 °C, produces more resistance to abrasion and consequently less weight loss of the anodized coating. This may be related to the fact that the residual stress of the anodized film formed at 5 Dc is higher than that formed at 20 °C. -
237
25
50
75
~J0
401-’-
125
Sealing Temperature VC)
I
(a)
_J
(h)
Fig. 5. (a) Effect on sealing temperature on weight loss (electrolyte S012 with 3 A d~n 2 at 20 °C for 30 mm; coatings sealed for 30 mm). (b) Film deterioration due to r~oorly sealed coating (magnification x 264). I
I
120
I
I
I
-
Cycles
(elOO)
Fig. 6. Abrasion resistance of anodized Al6ZnlMg (abraser load, 250 gf) in terms of 2 for 30 mm). weight loss (electrolyte 0503 with 4 A dm
Electrochemical polarizations were applied to evaluate the pitting resistance of Al6ZnlMg in fresh sea water. The chemical composition of sea water is shown in Table 2. The effect of sealing time on the passive current density is shown in Fig. 7. The anodic polarization curve for an unanodized specimen is also shown for comparison. Each specimen was anodized in bath 0503 with 4 A dm~2at 20 °Cfor 30 mm prior to sealing to produce a 25 jim coating. It is obvious that the effect of sealing time on the anodic behavior of this alloy is important, i.e. the longer the sealing time, the more pitting resistant the oxide film. A16Zn1Mg with zero sealing time possesses a discernible range of passivity which breaks down by pitting at —0.5 V(SCE) as shown in Fig. 7. Furthermore, sealing for 10 mm can effectively suppress the rapid pitting of the unsealed coating. The extent of anodic polarization
238 TABLE 2 The chemical composition of natural sea watera 2~
Ion species
Na~
CU
Mg
Concentration (M)
0.456
0.5339
0.0537
SO~2
K~
Ca2~
HC0
0.0272
0.0098
0.0693
0.0023
3
apH 8.2; specific gravity, 1.026.
0~IIIII~II~
IIIIII~II
I’~l~l
II~~I I
2 Current derluity ( A/cm Fig. 7. Effect of sealing time on the anodic behavior of anodized Al6ZnlMg (electrolyte 0503 with 4 A dm~2at 20 °Cfor 30 mm).
is apparently quite sensitive to the degree of sealing. Sealing for 20 and 30 mm further lowers the passive current densities. Figure 8(a) shows the effect of anodizing time on the anodic behavior of Al6ZnlMg in the same bath (0503) at 20 °Cand 4 A dm2. Each anodized coupon was sealed in boiling deionized water at 100 °Cfor 20 mm before its anodic polarization in aerated sea water. A similar decay of passive current density was also observed as the time for anodizing increased, as shown in Fig. 8(b). It is readily seen that a longer anodizing time as well as sealing time decreases the passive current densities. The anodized coating on aluminum is generally considered to have a duplex structure, i.e. a dense compact barrier layer next to the metal is surmounted by an outer porous layer. The porous layer has long been considered as consisting of oxide penetrated in a regular manner by columnar pores extending from the outer surface to the porous-barrier interface. However, in this context, the barrier layer itself provides little resistance to chloride penetration during anodic polarization, since the unsealed coatings were readily pitted. This result agrees with the previous mechanism of anodizing. Furthermore, the sealed oxide structure provides a strong resistance to pitting and anodic current flow. Therefore, anodic polarization is a good indicator and prediction of the resistance of the coating to breakdown by
239
JmIn8mIn5mI~,,,//)3mIn
2
(a)
Current density CA/cm .3
(b)
I
_____________________________________________________
Anodizing Time (mill.
Fig. 8. (a)Effectofanodizingtimeonthe anodic behavior of Al6AnlMg (electrolyte 0503 at 20 °C and 4 A dm2). For anodizing times of 10 mm, 8 mm, 5 mm and 3 mm, the film thickness was 9 pm, 7 pm, 4 pm and 2 pm respectively. (b) Effect of anodizing time on the passive current density of (a) at ÷1.6 V (SCE).
natural sea water. Jones [17] also demonstrated that the anodic polarization behavior of anodized coatings parallels their performance in atmospheric service and could be used as a test for the atmospheric corrosion resistance of anodized coatings. Pitting corrosion, as shown in Fig. 9, is confined to weak sites in the anodic coating which are usually of random distribution and not clearly connected with known metallurgical features, although the form of the intermetallic constituent is obviously connected with them because pitting decreases as the purity of the metal is increased. This is frequently the mode of deterioration of a well-sealed coating, the frequency of pitting being an inverse function of the thickness of the coating. It is also typical of marine environments which in themselves are not particularly hostile towards the
240
aluminum oxide film. Industrial conditions can of course be selectively aggressive in the same way. The fatigue resistance of A16Zn1Mg related to the different surface conditions is outlined in Table 3. These results are also presented in Fig. 10 in which heat treatment 4, i.e. the aged and anodized A16Zn1Mg, possesses the highest resistance to fatigue compared with the other surface treatments investigated. TABLE 3 Fatigue resistance of Al6ZnlMg after various treatments Heat treatment
Anodizing
Cycles before fracture at 40 ksi~
1. Annealed at 413 °Cfor 2.5 h and air cooled 2. Aged at 135 °C for iO~mm 3. Annealed at 413 °C for 2.5 h and air cooled 4. Aged at 135 °C for iO~mm
None
3.5
X
iO~
None
4.6
x
iO~
In 0503 at 20 °C, 2for 4 A dm~ 30 mm In 0503 at 20 °C, 4 A dm2 for 30 mm
6.0
x
8.0
x
*
Kilo pound per square inch.
~s0~_
Number of Cycles
Fig. 9. SEM photograph of the unsealed coating on Al6ZnlMg anodized in electrolyte 0503 with 4 A dm2 at 20 °Cfor 30 mm (magnification x273). Fig. 10. Plots of stress vs. number of cycles (S—N curves) for Al6AnlMg subjected to cyclic stress: 0, fully annealed; X, aged at 135 °Cfor iO~mm; A, anodized; •, aged and anodized.
241
4. Conclusion In summary, it can be stated that the resistance to general corrosion as well as pitting corrosion of A16Zn1Mg is significantly improved after anodizing in an appropriate bath. Anodizing at lower temperatures was found to give better resistance to wear. Judging from the limited data obtained so far, it appears that anodized AI6Zn1Mg possesses a better resistance to fatigue than the untreated alloy. The optimum anodizing conditions for this particular alloy as found in this study are a 5% oxalic acid plus 3% formic acid (electrolyte 0503) bath with a current density of 4 A dm2 for 30 mm at 20 °C.The resulting microhardness is 660 kg mm2 which is equivalent to that of hard chromium plating and the Taber Abraser Index appraoches 75.
Acknowledgment The authors wish to thank Professor K. S. Liu of the National Tsing Hua University for providing the Al—6%Zn—1%Mg alloy and for helpful discussions.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K. S. Liu, J. L. Horng and T. B. Wu, Chin. J. Mater. Sci., 11(1) (1979) 63. J. W. Yeh and K. S. Liu, Scr. Metall., 20 (3) (1986) 329. H. C. Shih and J. Y. Wu, Chin. J. Mater. Sci., 11(4) (1979) 85. C. H. Yu, MS. Thesis, National Tsing Hua University, 1979. E. Mattsson, L. 0. Gullman, L. Knutsson, R. Sundberg and B. Thundal, Br. Corros. J., 6 (1971) 73. 5. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and Its Alloys, Draper, Teddington, 4th edn., 1 (1972) 454. F. Keller, M. S. Hunter and D. L. Robinson, J. Electrochem. Soc., 100 (1953) 411. C. E. Michelson, J. Electrochem. Soc., 115 (1968) 213. A. J. Maeland, R. C. Rittenhouse and K. Bird, J. Plating Surf. Finishing, 63 (1976) 56. G. C. Tu, Chin. J. Mater. Sci., 18 (2) (1986) 90. K. Wefers and W. T. Evans, J. Plating Surf. Finishing, 62 (1975) 951. G. C. Wood, J. P. 0’SullivanandB.Vaszko,J. Electrochem. Soc., 115 (6) (1968)618. W. J. Bernard and J. W. Cook, J. Electrochem. Soc., 106 (8) (1959) 643. J. Zahavi and M. Metzger, J. Electrochem. Soc., 119 (11) (1972) 1479. R. K. Hart, Trans. Faraday Soc., 50 (1954) 269. N. D. Greene, Experimental Electrode Kinetics, Rensselaer Polytechnic Institute, Troy, NY, 1965. D. A. Jones, Corrosion (Houston), 25 (4) (1969) 187.