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Aluminium-magnesium alloys as corrosion resistant coatings for steel
Corrosion Science, Vol. 38, No. 1, pp. 155-170. 1996
Pergamon
Elsevier Science Ltd Printed in Great Britain 001&938X/96 $15.00+0.00
0010-938X(96)00123-9
ALUMINIUM-MAGNESIUM ALLOYS AS CORROSION RESISTANT COATINGS FOR STEEL K.R. BALDWIN,’
R.I. BATES,* R.D. ARNELL*
and C.J.E. SMITH’
I Structural Materials Centre, Defence Research Agency, Farnborough, Hampshire GUI4 6TD, U.K. ’ Research Institute for Design, Manufacture and Marketing, University of Salford, Salford M5 4WT, U.K Abstract-The corrosion behaviour of binary aluminium-magnesium (Al-Mg) alloy coatings deposited on to steel panels using an unbalanced magnetron sputtering technique has been studied. Neutral salt fog tests have shown that the corrosion resistance of sputtered aluminium alloy coatings increases as the magnesium content is raised until an optimum is reached for alloys containing approximately 20 wt% Mg. Increasing the magnesium content above 20wt% did not give further improvements in protection but instead caused a decline in corrosion performance. Potentiodynamic cathodic polarization sweeps conducted in 600 mmol 1-l sodium chloride solution have shown that the corrosion current density of sputtered aluminium coatings is increased as the magnesium content is increased, suggesting that there is a reduction in barrier protection afforded by the Al coatings as the Mg level is raised. Open-circuit corrosion potential measurements have shown that the sputtered aluminium alloys become progressively more active as the magnesium content is increased, suggesting that they afford improved sacrificial protection as the Mg level is raised. It is proposed that the peak in performance observed in the neutral salt fog environment represents an optimum balance between the barrier and sacrificial properties of the Al-Mg coatings. Keywords: A. aluminium,
A. magnesium,
A. metal coatings,
B. polarization,
C. sputtered
films.
INTRODUCTION Aluminium coatings are used on an industrial scale and can be applied using a variety of methods including metal spraying, hot dipping or cladding. Where fine tolerances are required, aluminium coatings can be deposited by electrodeposition from organic electrolytes1-6 or, more usually, by ion-vapour deposition (IVD) or other vacuum techniques.7-‘3 Since the early 1970s aluminium coatings deposited by the IVD route have been produced commercially and the technology required to obtain controlled thicknesses on both simple and complex shapes is well established. Aluminium coatings are now routinely employed for corrosion protection, decoration of metallic and non-metallic surfaces and have been suggested as a means of cladding less corrosion resistant aluminium alloy substrates.14 A further advantage of aluminium coatings is that they are receptive to conventional chromate passivation treatments. Aluminium coatings are used for the corrosion protection of ferrous substrates for two main reasons. Firstly, aluminium with its air-formed passive oxide film forms an effective barrier layer against attack and, secondly, if the underlying substrate is exposed at, for example, the base of pre-existing defects or corrosion pits, the aluminium coating will afford a degree of sacrificial protection to the substrate. A further advantage with aluminium Manuscript received 0 Crown copyright
1 June 1995
(I 995). 155
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coatings is that their corrosion potentials are relatively close to those of many structural aerospace aluminium alloys and hence there is very little driving force for galvanic corrosion between them.i5-” Thus the use of aluminium coatings on, for example, steel fasteners, will serve to reduce bimetallic corrosion between steel components and the structural aluminium alloys with which they may come into contact. A disadvantage of aluminium coatings deposited by IVD is that they are often columnar and porous in nature.‘-12 This porous structure may result in inefficient barrier protection and the premature breakdown of the coating since the substrate is rapidly exposed to the corrosive environment at the base of the pores. The porosity of IVD aluminium coatings can be reduced by compacting the surface with post-plating glass-bead peening treatments. Electrolytically deposited aluminium coatings are generally more dense than as-plated IVD aluminium and thus the former may afford high levels of protection to steel substrates when exposed to aggressive chloride bearing environments.i6*‘* Recent advances at the University of Salford have enabled aluminium coatings to be obtained using unbalanced magnetron sputtering.” Aluminium coatings deposited using this vacuum technique are dense and non-columnar with significantly improved barrier properties over those produced by conventional ion-plating. Magnetron-sputtering can be used to deposit not only pure metals such as aluminium but also alloy coatings with controlled compositions. A drawback with pure aluminium is that since its corrosion potential is not far removed from that of many steels, it is unable to provide adequate levels of sacrificial protection, particularly under natural exposure conditions.2s22 Earlier work at DRA Farnborough2’ has shown that if aluminium is alloyed with small amounts of active metals such as magnesium, its corrosion potential can be depressed, leading to a significant improvement in sacrificial properties. In the present work, an unbalanced magnetron sputtering technique has been used to deposit Al-Mg alloys with both improved barrier and sacrificial properties. Aluminium alloys containing up to 50% Mg and above have been deposited on to steel substrates and the corrosion behaviour of the sputtered deposits determined using neutral salt fog tests and electrochemical techniques. Particular attention has been paid to determining the effects of magnesium additions on the barrier and sacrificial properties of aluminium.
EXPERIMENTAL
METHODS
The production of coatings Pure Al and Al-Mg alloy coatings were deposited using an unbalanced magnetronsputtering technique at the University of Salford. The use of magnetron-sputtering has been reviewed in several published articles23-25 and precise details of the unit and its operation in the present work are described elsewhere.” In the present work, mild steel test panels of 1 mm thickness (50 x 50 mm dimensions) were initially degreased using an ultrasonic bath containing alcohol and then placed in the vacuum chamber. The magnetron sputtering chamber was evacuated to lo-’ Torr or below and then backfilled with high purity argon to typically 5 x lop3 Torr. The chamber contained two consumable targets, one of commercial grade aluminium (99.4% purity) and one of commercial grade magnesium (99.5% purity). Prior to deposition, the steel test panels were sputter cleaned, typically at an applied voltage of 1 kV for 30 min, to remove any traces of surface contamination. Following this high voltage sputter cleaning process, a low-voltage was applied to the test
Aluminium-magnesium alloys
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panels, typically - 50 V (DC), to provide an external bias during deposition. Unbalanced magnetron sputtering utilises the plasma in front of the metal targets to self bias the substrates during deposition. In the present work, this was achieved by weakening the magnetic field strengths of the targets and allowing the plasma to escape the confines of the target surface and engulf the substrates. However, due to the large substrate assembly, in relation to the size of the targets, and the fact that the targets are only slightly unbalanced, the coating facility at Salford necessitates the use of an external bias supply during deposition. The targets were energised using an electrical current, to typical power levels of 3 kW for the aluminium target and 1.5 kW for magnesium target. The power levels were varied to obtain the required ratio of Al:Mg in the deposits by producing different sputtering rates. The coating process was continued for a sufficient time to allow the required thickness of deposit to build up on the substrates. After the required thickness had been obtained, the deposition process was halted and the coupons allowed to cool under vacuum to prevent oxidation. A typical deposition rate for the pure Al and Al-Mg alloy coatings was 0.3 pm min-’ , depending on the power supplied to the targets. The composition of each alloy coating was checked using electron-microprobe analysis. The sputtering process was used to produce coatings with thicknesses of 5, 7.5, 10, 15, 25 and 2.5pm. The thickness of each coating was measured prior to testing using a nondestructive eddy-current method. The coatings were not subjected to any post-plating treatments and were tested in the as-plated condition only. Neutral salt fog tests
The corrosion resistance of as-plated pure Al and Al-Mg alloy coatings containing up to 60 wt% Mg were evaluated using neutral salt fog tests conducted in accordance with ASTM Bl 17.26The salt fog cabinet was fed from a reservoir containing 5% w/v NaCl solution with no pH adjustment. The NaCl solution was pumped to an atomizing jet and compressed air directed into the stream of solution to produce a dense salt fog. The test panels were mounted on Perspex racks at an angle of 20” to the vertical and inspected daily for signs of corrosion. Open-circuit
potential
measurements
Open-circuit corrosion potential measurements were conducted using test electrodes constructed from as-plated pure Al and Al-Mg alloy coatings of 25 pm thickness on steel substrates, The test electrodes were separately immersed in 600 mmol 1-l (3.5% w/v) NaCl solution (100 ml) contained in polypropylene beakers. The beakers were positioned in a water bath maintained at 25 f 2°C. A Luggin capillary, in solution contact with an external reference electrode (SCE), was positioned adjacent to the surface of each test electrode and the potential measured at 30 min intervals using a programmable voltmeter. The experiments were halted after 200 h in each case. Polarization
sweeps
Cathodic potentiodynamic polarization sweeps were carried out in a conventional threeelectrode glass cell using test electrodes constructed from sputtered pure Al and Al-Mg alloy coatings of 25 pm thickness. The test electrodes were immersed in 600 mmol 1-l NaCl solution at 25 + 1“C contained in the glass cell. Prior to each sweep, the test electrodes were permitted to stabilise for approximately 30 min. Immediately following the stabilisation period, the electrodes were polarized cathodically at a sweep rate of 0.125 mV SF’. The
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sweeps were conducted experiment.
in solutions
et al.
which were gently aerated
EXPERIMENTAL
both before and during
the
RESULTS
Coating microstructure The microstructure of magnetron-sputtered pure Al and Al-Mg alloy coatings was investigated using scanning electron microscopy (SEM). Figure l(a) shows the surface of a pure Al sputter deposit on a steel substrate. The pure Al coating was flat and almost completely featureless which is typical of the dense, non-columnar, coatings which can be deposited by unbalanced magnetron sputtering.” The pure Al coating appeared to afford complete coverage to the substrate and was almost completely free of holidays (small pores) or other microscopic surface defects.
(a) Pm Aluminium
(d) k-57 wt% r&g
(c) Al-32 wt% Mg w Fig.
1. SEM micrographs of fracture cross-sections of unbalanced magnetron and AI-Mg alloy coatings on steel substrates. The black bar at the bottom
sputtered pure Al is 4 pm long.
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alloys
Figures l(b)-l(d) show fracture cross-sections of Al alloy coatings containing 5, 32 and 57 wt% Mg. The Al-Mg alloy coatings were similar to pure Al in that the sputter deposits were flat and generally featureless indicating that the Mg content had very little effect on the microstructure of the coatings. The pure Al and Al-Mg alloy coatings exhibited extremely good adhesion to the steel substrate. Neutral salt fog tests
The corrosion of the sputtered pure Al and Al-Mg alloy deposits containing up to 30 wt% Mg in the salt fog environment was initially characterized by the formation of large pits in the coating surface, although the coatings remained generally bright. White corrosion products, commonly termed white-rust, were found to build-up in the region of the pits and this was usually accompanied by the formation of gelatinous corrosion products. The Al alloy coatings containing over 30 wt% Mg were generally less susceptible to localized pitting attack but instead appeared to corrode in a more uniform manner. With prolonged exposure to the salt fog, the coatings ceased to provide full protection against attack and corrosion of the underlying metal occurred, giving rise to the formation of red-rust, the iron oxide (FezOs) corrosion products of the ferrous substrate. Figure 2 illustrates the time taken for the appearance of first red-rust as a function of alloy Mg content for 8 pm thick coatings. It was found that the time to red-rust increased as the alloy Mg content was increased until a peak in corrosion performance was obtained for Al alloys containing approximately 20 wt% Mg. Further increases in the magnesium content were of no further benefit but instead caused a decrease in the corrosion resistance of the Al-Mg
I
10 Fig. 2. The relationship magnetron sputtered
I
I
I
30 40 20 Ut X Mg alloying addition
I
50
between time to red-rust and alloy composition for 8 pm thick unbalanced AI-Mg alloy coatings on steel-exposed to continuous neutral salt fog.
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alloy coatings. Figure 2 shows that the sputtered Aiwt% Mg alloy delayed the onset of red-rust for almost twice as long as the pure aluminium coatings. The effect of coating thickness on time to red-rust for sputtered pure Al and Al alloys containing 5, 10, 15, 30 and 55 wt% magnesium is given in Fig. 3. For pure aluminium coatings, the time to red-rust increased linearly as the thickness was increased, which is in good agreement with earlier work’6,18 on the corrosion behaviour of IVD pure Al coatings in the neutral salt fog environment. Figure 3 shows that, in contrast with the pure Al coatings, for the Al-Mg alloys as the Mg level was increased up to 30 wt%, a significant deviation from linearity occurred in the thickness-time to red-rust relationship. This was particularly pronounced for the Al-30 wt% Mg alloy coatings, where the time to red-rust increased sharply for a relatively small increase in coating thickness. When the concentration of magnesium metal was increased to 55 wt%, a return to linear behaviour was obtained. Information relating to the kinetics of coating breakdown can be obtained by determining the rate at which red-rust forms on the panel surfaces once full barrier and sacrificial protection have been lost. The relationship between the area of red-rust and exposure time for 8pm thick sputtered coatings is shown by Fig. 4. For pure Al and the Al alloys containing up to 35 wt% magnesium, the spread of red-rust was limited, with the area of rust increasingly very slowly for extended exposure periods. Following this initial period, where the coating remained largely intact, the rate of coating loss increased sharply with the
Time
tD red-rust
Ihours)
Fig. 3. The relationship between coating thickness and time to red-rust for unbalanced sputtered pure Al and Al-Mg alloy coatings on steel-exposed to continuous neutral
magnetron salt fog.
Aluminium-magnesium
Exposure
time
alloys
161
(hours)
Fig. 4. The relationship between area of red-rust and exposure time for unbalanced sputtered pure Al and AI-Mg alloy coatings on steel-exposed to continuous neutral
magnetron salt fog.
sputtered deposits being rapidly consumed. It is possible that this type of behaviour was related to the dense nature of the sputtered deposits. The coatings may have become very thin before a significant area of the substrate became exposed to the corrodant so that rapid corrosion of the underlying steel would have only occurred at the moment when the coating was finally lost. Figure 4 shows that the Al-55 wt% Mg alloy coating was consumed rapidly once red-rust had initiated, reflecting the poor corrosion resistance of this high-Mg alloy. Open circuit potential
measurements
The variation in open circuit corrosion potential, EC,,,, with time for sputtered pure Al and the alloys containing 5, 10, 1.5, 25 and 55 wt% Mg, recorded during immersion in quiescent 600 mmol 1-l NaCl solution, is shown by Fig. 5. The test electrode constructed from pure Al initially adopted a potential of - 0.90 V (SCE) in the chloride solution. The potential then increased sharply over the first few hours of immersion to a value of approximately - 0.74 V (SCE) which is similar to that previously recorded for ion-plated aluminium in quiescent sodium chloride solutions.16*18The corrosion behaviour of the pure Al coatings was characterized by frequent potential fluctuations of up to 0.1 V in magnitude, which are usually attributed to the effects of localized corrosion such as pitting attack.27-30 Figure 5 shows that for aluminium alloys containing the lower magnesium contents of 15 wt% and below, similar potential-time behaviour was observed to that obtained for sputtered pure aluminium, with a steep rise in potential being followed by a period of rapid potential fluctuations. In contrast, when the alloy magnesium content was further increased to 25 wt%, the open circuit potential was generally more stable with the fluctuations in potential being less pronounced. This suggests that the Al alloys containing moderate levels of magnesium may be more resistant to pitting attack than pure Al or those containing low levels of Mg, supporting the observations made in the salt fog environment. This trend was continued with the aluminium alloy containing 55 wt% magnesium, where very few potential fluctuations were observed. However, the Al-55 wt% Mg alloy behaved differently from the alloys which were mainly composed of Al in that following a very large rise in open-circuit potential in the first 100 h of immersion, the potential did not stabilise but continued to drift in a noble direction.
er al.
K.R. Baldwin
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Pure Aluminium
4
(D
-1.01 0
100 Tlme (hours1
Time
fhoursl
v .
Al-5% Mg
m ,
,
,
,
,
,
Time Ihours)
Time lhoursl
jg
-0.6
200
Time lhoursl
Time Ihours)
Fig. 5. The variation in open circuit corrosion potential with time for unbalanced magnetron sputtered pure Al and AI-Mg alloy coatings containing 5, IO. 15, 25 and 55 wt% Mg, determined in quiescent 600 mmol I-’ sodium chloride solution.
The average open-circuit corrosion potential of the sputtered deposits was calculated over the 200 h immersion period in 600 mmol 1-t NaCl solution. Figure 6 illustrates the relationship between the average open-circuit corrosion potential and Al-Mg alloy composition. This plot shows that the average potential of the Al alloys became increasingly negative as the Mg content was increased, which is in line with a previous study on Al-Mg alloys carried out at DRA Farnborough.” The open-circuit corrosion potential of bare steel has been determined in earlier workI and a value of -0.67 V (SCE)
Aluminium-magnesium
iii
-0.6
ti -0.7
a, : -1.0 & z
163
alloys
r___-___-___________-~---
EeOPr tsteell-------
-
-1.1-
I 0
I
I
10
20
WtX Mg
alloying
I
30 addition
I
40
I
50
Fig. 6. The relationship between average open circuit corrosion potential and alloy composition for unbalanced magnetron sputtered AI-Mg alloy coatings, determined over 200 h immersion in 600 mmol 1-l NaCl solution.
was obtained in quiescent 600 mmol 1-l NaCl solution. Figure 6 shows that the sputtered deposits were negative with respect to bare steel and that the Al alloys will become increasingly sacrificial as the concentration of the more active Mg metal is increased. Potentiodynamic
polarization
sweeps
The cathodic polarization sweeps obtained for magnetron-sputtered pure Al and selected Al-Mg alloys in aerated 600 mmol 1-l NaCl solution are shown by Fig. 7. For the pure Al coatings, a rapid increase in current density was initially observed close to the opencircuit potential. The cathodic response then showed highly polarized behaviour, with only a very small increase in current density being observed for a large increase in overpotential. This behaviour is typical of corrosion processes that are under diffusion control, e.g. oxygen reduction.31 Figure 7 shows that the cathodic polarization response of the Al-Mg alloys was similar to that obtained for pure Al in that diffusion controlled cathodic reactions appeared to be predominant. However, it was found that the polarization curves obtained for the Al-Mg alloy coatings were, in most cases, less polarized than pure aluminium, with the alloys showing some deviation from purely diffusion limited behaviour, particularly at the higher overpotentials. A further effect of increasing the alloy magnesium content was to shift the cathodic polarization curves to progressively higher current densities. The corrosion current density was determined for each sputtered deposit using a Tafel extrapolation technique.32 The corrosion current density values obtained are plotted logarithmically as a function of magnesium content by Fig. 8. This plot shows that the corrosion current density of the Al alloys increased as the Mg content was increased and suggests that the barrier corrosion resistance of sputtered Al will be progressively diminished as the concentration of Mg in the alloys is raised.
K.R.
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-7oc
-900 z 5: : >
M9 .5x M9 -1100
E r( II) .?I JJ s &J -1300 a0
J
-1500 I
-3
-2
Log.Current
-1 Density
0
i
2
$tA/mmz)
Fig. 7. Cathodic potentiodynamic polarization sweeps of unbalanced magneton sputtered pure Al and AI-Mg alloy coatings in aerated 600mmoll-’ sodium chloride solution. Sweep rate: 0.125mVs-‘.
Figure 7 showed that as the alloy magnesium content was increased the cathodic polarization curves were shifted to lower potentials. The relationship between the corrosion current density and the open circuit potentials of the sputtered deposits determined after 1 h immersion in aerated 600 mmol I-‘NaCl solution is shown by Fig. 9. The plot obtained for the Al-Mg alloys indicates that the corrosion current density increased as the open circuit potential became more negative. This type of relationship has been observed for other binary alloy systems33*34 and demonstrates that the corrosion of the Al-Mg alloys was predominantly under cathodic control since the corrosion current became larger as the potential became more negative.
DISCUSSION Barrier properties
of coatings
The cathodic polarization sweeps conducted on sputtered pure Al showed the type of highly polarized behaviour normally associated with corrosion reactions that are under diffusion control (Fig. 7). In the near neutral solutions employed in this study, the predominant cathodic reaction is considered to be oxygen reduction. The cathodic corrosion behaviour of the sputtered Al alloys was similar to that of pure Al even for those alloys containing high Mg levels. For example, the Al-55wt% Mg alloy behaved in a similar manner to pure Al in that diffusion control was predominant, rather than showing
Aluminium-magnesium
alloys
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0.
Magnesium Fig. 8.
content
(wtX1
The relationship between corrosion current density and magnesium content for unbalanced magnetron sputtered AI-Mg alloy coatings in aerated 600 mmol I-’ NaCl solution.
the more activebehaviour associated with pure magnesium where hydrogen evolution is often the principal cathodic process.3S37 The cathodic polarization sweeps showed that the corrosion current density of pure Al was shifted to higher currents as the Mg level was raised (Figs 7 and 8). If the cathodic current was determined solely by the diffusion limited reduction of oxygen, then no effect of the nature of the cathode should be observed and the corrosion current should be independent of alloy composition. The fact that the currents were very different shows that the nature of the alloy surface was significant. The addition to aluminium of significant concentrations of magnesium would certainly be expected to change the nature of the passive surface film on Al. In the presence of moisture, Mg develops a thin film of magnesium hydroxide, Mg(OH)2, which is generally incomplete and provides only limited protection to magnesium. Thus the addition of sufficient concentrations of Mg to Al would be expected to disrupt the surface oxide film formed on Al and weaken its protective effect, possibly accounting for the poorer barrier properties of the Al-Mg binary alloys. The increase in the cathodic current that occurs as the magnesium content is raised may be related to the increased rate of oxygen diffusion through the less protective oxide film formed on the Al-Mg alloys. Mansfeld et a1.38 employed similar arguments to explain the difference in corrosion rates between aluminium alloys where none should be found since their corrosion reactions were under cathodic control. Mansfeld et al.38 also proposed that differences in efficiency for oxygen reduction on individual Al alloys themselves could also account for the observed difference in their corrosion rates.
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Open-circuit
potential
(V vs SCEl
Fig. 9. The relationship between corrosion current density and open circuit corrosion potential for unbalanced magnetron sputtered AI-Mg alloys determined after 1h immersion in aerated 600 mmol I-’ NaCl solution. Weight percentage magnesium content ofeach alloy shown in brackets.
The barrier corrosion resistance of the Al coatings will not only be dependent on the rate at which they corrode but also the nature of the corrosion itself. In the neutral salt fog environment it was found that the corrosion of the Al and Al-Mg alloy coatings was characterized mainly by pitting attack. This type of localized attack is thought to result from the presence of flaws in the passive oxide film. In studies on pure Al coatings, Hinton et al.” proposed that if the local microstructure exhibited little or no porosity, attack occurred mainly by uniform penetration over a wide area, while some pitting was prevalent at grain boundaries. With as-plated ion-vapour deposited coatings, Hinton et al.” found that the local microstructure contained high levels of porosity, and was thus characterized mainly by localized pitting, leading to early exposure of the substrate. In the present work, the corrosion of the dense, non-porous sputtered Al alloys containing low Mg levels (< 15 wt%) was characterized by the formation of large pits similar in form to those observed for pure Al. The susceptibility of these coatings to pitting attack was demonstrated by the open circuit corrosion potential measurements which showed potential fluctuations which are typical of this type of localized attack. In contrast, those Al-Mg alloys containing higher Mg levels were less susceptible to pitting attack. A possible explanation is that the high Mg alloys were gradually assuming the characteristics of pure Mg for which a more uniform by the open circuit potential type of attack is often found.36337 This was supported measurements which showed a marked increase in electrochemical stability as the alloy Mg level was increased (Fig. 5).
Aluminium-magnesium
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For pure Al and the Al-Mg alloy coatings, the air-formed film will provide the first barrier against attack in the presence of an aggressive environment. In the presence of chloride ions, appreciable thinning of the oxide film is likely to occur, and eventually the underlying metallic layer will be exposed to the corrodant. The corrodant will penetrate preexisting defects in the coating such as small pores, which will then behave as crevices. In the pores or in pits formed as a result of corrosion, the local environment will become increasingly acidic. In the case of pure Al, the development of local acidity will accelerate the dissolution of Al and thus stimulate pit growth.39 For the Al-Mg alloys, the development of strongly acidic conditions may lead to the dissolution of both Al and Mg. In the early stages of pit growth, hydrogen evolution would occur because of the limited oxygen supply and the low solubility of oxygen in solutions of high chloride concentration. lo Following this initial period, the pH in the pit is thought to rise as oxygen reduction becomes the predominant cathodic reaction thus generating hydroxyl ions. In the case of pure Al, hydroxides such as Al(OH)s, then precipitate in the pit, accompanied by the formation of other insoluble corrosion products such as Mg(OH)2 in the case of the Al-Mg alloys which may help to stifle corrosion activity. The sacrificialproperties
of coatings
The electrochemical potential of binary alloys is related to the thermodynamic equilibrium potentials of the two components of the alloy.40 The electrochemical potential adopted by the binary alloy will be located between the potential values of the parent metals. The potential of pure magnesium is approximately - 1.9 V (SCE) in 600 mmol 1-l chloride solution35-37 compared with a value of between -0.7 and - 1.OV (SCE) for aluminium, depending upon its purity.‘5-18 Accordingly, the addition of Mg to the sputtered Al deposits produced alloys which adopted open circuit corrosion potentials that were negative to that of pure Al but were positive with respect to that of pure Mg (Fig. 6). Previous work at DRA Farnborough4i has shown that open circuit potential measurements can be a reliable guide in predicting the direction of current flow in a galvanic couple formed between metal coatings and steel although the magnitude of the current cannot be determined solely from potential data and must usually be determined experimentally. If the potential of the coating is negative to steel then sacrificial protection will be obtained whereas if it is more noble than the substrate, the steel will corrode preferentially. In the present work, the open circuit potential adopted by pure Al was close to that of bare steel which, as indicated above, adopts a potential of -0.67 V (SCE) 600 mmol 1-l NaCl solution.16 Although pure Al is slightly negative to steel, it is possible that the coatings will be insufficiently active to drive the potential of the steel substrate well below its protection potential, accounting for the limited sacrificial protection afforded by the pure Al. The sputter deposited Al alloy coatings were found to become increasingly active as the magnesium content was raised, indicating that the coatings would afford improved sacrificial protection to steel over that obtained for pure Al. Figures 5 and 6 showed that alloying additions of 15 wt% Mg and above are required to obtain the significant negative shift in potential to ensure that steel would be effectively protected at any pre-existing pores or corrosion pits in the coating. If the magnesium level was increased significantly above this value, to over 30 wt% Mg, the potential of the alloys was found to decrease below - 1.OV (SCE). Although Al-Mg alloys which adopt such active potentials would provide very effective sacrificial protection, it is likely that they would not be present for long since they are likely to suffer high rates of galvanic corrosion.
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Binary alloy systems composed of two metals with widely different electrochemical potentials are often susceptible to de-alloying. This occurs with, for example, some types of copper-zinc alloys (brasses) where the more active metal (Zn) selectively dissolves from the alloy matrix leaving a deposit enriched in the more noble metal (Cu). In simple terms, the Al alloys containing less than 50 wt% magnesium are analogous to the Cu-Zn alloys in that the more active metal (Mg) is contained in a matrix composed of the more noble metal (Al). An earlier study conducted at DRA Farnborough42 has shown that open circuit potential monitoring is useful in studying de-alloying since a significant positive shift in potential generally occurs as the more active component of the alloy is dissolved from the alloy surface. In the present work, the potential of the Al alloys containing less than 50 wt% Mg remained relatively stable despite the fluctuations in potential, suggesting that the Al-Mg alloys which were composed mainly of Al were resistant to de-alloying. The open circuit corrosion potential measurements conducted on the sputtered alloys composed mainly of Mg were found to behave differently to those composed mainly of Al. Fig. 5 shows that the potential of the Al-55 wt% Mg alloy increased sharply over the initial immersion period. In contrast to pure Al and the low-magnesium alloys which also showed initial rises in potential, the Al-55 wt% Mg alloy became increasingly noble and did not stabilise suggesting that it is susceptible to a form of de-alloying. A possible explanation is that magnesium was selectively dissolved leaving a surface enriched in the more noble aluminium, accounting for the noble shift in potential. Toward the end of the immersion period, the potential of the high-magnesium alloy did eventually show signs of stabilising, possibly as an equilibrium structure developed in the surface layers. Corrosion behaviour iiz neutral salt f5g In the neutral salt fog tests, it was found that the level of corrosion protection afforded to steel by the sputtered Al coatings increased sharply as the magnesium content was increased until a peak in performance was observed for alloys containing 20 wt% magnesium. Further increases in the magnesium content were of no further benefit and instead caused a decline in corrosion performance. The cathodic polarization sweeps showed that the corrosion current density of Al in 600 mmol l- ’ NaCl solution was increased as the Mg level was raised. The values obtained for Al alloys with Mg concentrations of up to approximately 20 wt% were lower than corrosion currents previously recorded for ion-vapour deposited pure Al,’ where the corrosion current density of the as-plated IVD coatings in aerated 600 mmol 1-l was found to be approximately 0.01 PA mm-‘, which is 10 times higher than that recorded in the present work for sputtered pure Al under very similar experimental conditions. The significantly higher corrosion current density found for the ion-vapour deposited aluminium coating is most likely due to its high porosity compared with the sputtered deposits evaluated in this study. Thus it may be concluded that although the addition of significant concentrations of Mg would have reduced the barrier corrosion protection afforded by the Al alloy sputtered deposits, they would remain more effective than conventional ion-plated aluminium coatings provided the Mg content does not exceed approximately 20 wt %. In the neutral salt fog environment, it is proposed that the initial increase in the level of corrosion protection observed in the neutral salt fog environment for alloys containing up to 20 wt% magnesium was mainly due to an improvement in the level of sacrificial protection afforded by the sputtered deposits as the Mg concentration was increased. It has been shown that the loss of barrier properties would not have become a significant factor until the
Aluminium-magnesium
alloys
169
magnesium content was relatively high ( > 20 wt% Mg). The decline in the level of corrosion protection afforded by the sputtered Al-Mg alloys (Fig. 2) when the magnesium level was increased above 20 wt% was most likely due to the combined effects of excessive rates of galvanic corrosion as the coatings became more active and the loss of barrier corrosion protection. Thus the data obtained in the present work suggest that the peak in performance observed in the neutral salt fog environment for the sputtered deposits containing approximately 20 wt% Mg represents an optimum balance being attained between the barrier and sacrificial properties of the Al-Mg alloy coatings. CONCLUSIONS 1. Potentiodynamic cathodic polarization sweeps conducted on unbalanced magnetron sputtered Al-Mg alloy coatings in 600mmol I-’ NaCl solution showed that the corrosion current density of the Al alloys increased as the Mg content was raised, indicating that the barrier corrosion resistance of sputtered Al will decline as the alloy Mg level is increased. 2. From open circuit corrosion potential measurements conducted in 600 mmol l- ’ NaCl solution it was found that sputter deposited Al alloy coatings become progressively more active as the alloy Mg content is increased, suggesting that the Al alloys will become more sacrificial as the Mg level is raised. 3. In neutral salt fog tests, the level of corrosion protection afforded by Al alloy coatings is improved by the addition of Mg until an optimum is reached for alloys containing approximately 20 wt% Mg. Further increases in the Mg level, above 20 wt%, caused a decline in performance. It is proposed that the peak in performance represents an optimum balance attained in the neutral salt fog environment between the barrier and sacrificial properties of the Al-Mg alloy coatings. REFERENCES I G.A. Capuano, R. Lafortune and C. Gobeil, Plating and Surface Finishing &I,49 (1977). 2. R. Suchentrunk, Corrosion Protection by Electra-deposited Aluminium, AGARD Lecture Series 106, I. l-7.25 (1980). 3. W. Wittich. R. Suchentrunk and H. Kellerer, Metal1 und Technik 30, 943 (1980). 4. M. Galova, Surf. Technol. 11, 357 (1980). 5. S. Biallozor. A. Lisowska-Oleksiak and W.N. Titova, Surf Coat. Technol. 34, 549 (1988). 6. W. Kautek, Corros. Sci. 28, I73 (1988). 7. E. Fanin, AGARD Conf Proc., No. 256, Advanced Fabrication Processes, Florence, Italy, 2628 September (1978). 8. M. El-Sherbiny and E. Salem, Anti-corrosion 28, 15 (1981). 9. N.A.G. Ahmed, Anti-Corrosion 31, 4 (1984). 10. B.R.W. Hinton, W.J. Pollock, P.N. Trathen and A. Crosky, Corros. Australasia 13, 12 (1987). 11. R. Palmer and D.J. Hall, Product Finishing 45, 6 (1992). 12. B.T. Nevill, Plating Surf Finishing 80, 14 (1993). 13. J.F. McIntyre and L. Hang, Corrosion 50, 26 (1994). 14. J.C. Salvador Fernandes and M.G.S. Ferreira, Surf Coat. Technol. 53, 99 (1992). 15. W.H. Ailor, Proc. ConfI Corrosion ‘70, Paper No. 36, p. 200. Philadelphia (1970). 16. C.J.E. Smith and K.R. Baldwin, Product Finishing 45, 12 (1992). 17. T.D. Burleigh, R.C. Rennick and F.S. Bovard, Corrosion 49, 683 (1993). 18. K.R. Baldwin and C.J.E. Smith, Proc. Co@ Materials Technology ‘91 (ed. A. Niku-Lari), p. 153. IITTTechnology Transfer International, Paris, September (1991).
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19. D.P. Monaghan. D.G. Teer, P.A. Logan, K.C. Laing, R.I. Bates and R.D. Amell, Surf. Coat. Technol. 60, 592 (1993). 20. P.L. Lane and C.J.E. Smith, Surface Engineering Practise (ed. K.N. Strafford, P.K. Datta and J.S. Gray), p. 566. Ellis-Horwood Ltd., Chichester (1990). 21. J.B. Horton, Corrosion Control by Coatings (ed. H. Leidheiser), Science Press, p. 59. Princeton (1979). 22. V.C.R. McLaughlin. AGARD Cant Proc.. No. 256, Advanced Fabrication Processes, Florence, Italy, 2628 September (1978). 23. B. Window and N. Savvides, J. Var. Sri. Technol. A4, 196 (1986). 24. B. Window and N. Savvides, J. Var. Sci. Technol. A4, 453 (1986). 25. B. Window and N. Savvides, J. Var. Sri. Technol. A4, 504 (1986). 26. ASTM B117. Standard Method of Salt Spray (Fog) Testing, American Society for Testing and Materials, Philadelphia (1981). 27. M. Janik-Czachor, G.C. Wood and G.E. Thompson, Br. Corros. J. 15, 154 (1980). 28. B. Samuels, K. Sotoudeh and R.T. Foley, Corrosion 37, 92 (1981). 29. R.T. Foley, Corrosion 42, 277 (1986). 30. K. Nisancioglu and H. Holtan, Werkstoffe und Korrosion 30, 105 (1979). 31. M.G. Fontana and N.D. Greene, Corros. Eng. Ch. 9. McGraw-Hill Book Co., New York (1967). 32. F. Mansfeld, Electrochemical Techniques for Corrosion (ed. R. Baboian), p. 18. NACE (1977). 33. K.R. Baldwin, M.J. Robinson and C.J.E. Smith, Corros. Sri. 36, 1115 (1994). 34. N.R. Short, A. Abibsi and J.K. Dennis, Trans. Inst. Met. Finishing 67, 73 (1991). 35. F. Hehmann, F. Sommer, H. Jones and R.G.J. Edyvean, J. Mar. Sci. 24, 2369 (1989). 36. G.L. Makar and J. Kruger, J. electrochem. Sot. 137, 414 (1990). 37. G.L. Makar, J. Kruger and K. Sieradzki. J. electrochem. Sot. 139, 47 (1992). 38. F. Mansfeld, D.H. Hengstenberg and J.V. Kenkel, Corrosion 30, 343 (1974). 39. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Section 5.1. 2nd Edition. NACE; Cebelcor (1974). 40. H. Kaiser, Corrosion Mechanisms (ed. F. Mansfeld), p. 85. Marcel Dekker Inc. (1987). 41. K.R. Baldwin, M.J. Robinson and C.J.E. Smith. Br. Corrosion J. 29, 299 (1994). 42. K.R. Baldwin, M.J. Robinson and C.J.E. Smith. Corros. Sci. 35, 1267 (1993).
Pergamon
Elsevier Science Ltd Printed in Great Britain 001&938X/96 $15.00+0.00
0010-938X(96)00123-9
ALUMINIUM-MAGNESIUM ALLOYS AS CORROSION RESISTANT COATINGS FOR STEEL K.R. BALDWIN,’
R.I. BATES,* R.D. ARNELL*
and C.J.E. SMITH’
I Structural Materials Centre, Defence Research Agency, Farnborough, Hampshire GUI4 6TD, U.K. ’ Research Institute for Design, Manufacture and Marketing, University of Salford, Salford M5 4WT, U.K Abstract-The corrosion behaviour of binary aluminium-magnesium (Al-Mg) alloy coatings deposited on to steel panels using an unbalanced magnetron sputtering technique has been studied. Neutral salt fog tests have shown that the corrosion resistance of sputtered aluminium alloy coatings increases as the magnesium content is raised until an optimum is reached for alloys containing approximately 20 wt% Mg. Increasing the magnesium content above 20wt% did not give further improvements in protection but instead caused a decline in corrosion performance. Potentiodynamic cathodic polarization sweeps conducted in 600 mmol 1-l sodium chloride solution have shown that the corrosion current density of sputtered aluminium coatings is increased as the magnesium content is increased, suggesting that there is a reduction in barrier protection afforded by the Al coatings as the Mg level is raised. Open-circuit corrosion potential measurements have shown that the sputtered aluminium alloys become progressively more active as the magnesium content is increased, suggesting that they afford improved sacrificial protection as the Mg level is raised. It is proposed that the peak in performance observed in the neutral salt fog environment represents an optimum balance between the barrier and sacrificial properties of the Al-Mg coatings. Keywords: A. aluminium,
A. magnesium,
A. metal coatings,
B. polarization,
C. sputtered
films.
INTRODUCTION Aluminium coatings are used on an industrial scale and can be applied using a variety of methods including metal spraying, hot dipping or cladding. Where fine tolerances are required, aluminium coatings can be deposited by electrodeposition from organic electrolytes1-6 or, more usually, by ion-vapour deposition (IVD) or other vacuum techniques.7-‘3 Since the early 1970s aluminium coatings deposited by the IVD route have been produced commercially and the technology required to obtain controlled thicknesses on both simple and complex shapes is well established. Aluminium coatings are now routinely employed for corrosion protection, decoration of metallic and non-metallic surfaces and have been suggested as a means of cladding less corrosion resistant aluminium alloy substrates.14 A further advantage of aluminium coatings is that they are receptive to conventional chromate passivation treatments. Aluminium coatings are used for the corrosion protection of ferrous substrates for two main reasons. Firstly, aluminium with its air-formed passive oxide film forms an effective barrier layer against attack and, secondly, if the underlying substrate is exposed at, for example, the base of pre-existing defects or corrosion pits, the aluminium coating will afford a degree of sacrificial protection to the substrate. A further advantage with aluminium Manuscript received 0 Crown copyright
1 June 1995
(I 995). 155
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et al
coatings is that their corrosion potentials are relatively close to those of many structural aerospace aluminium alloys and hence there is very little driving force for galvanic corrosion between them.i5-” Thus the use of aluminium coatings on, for example, steel fasteners, will serve to reduce bimetallic corrosion between steel components and the structural aluminium alloys with which they may come into contact. A disadvantage of aluminium coatings deposited by IVD is that they are often columnar and porous in nature.‘-12 This porous structure may result in inefficient barrier protection and the premature breakdown of the coating since the substrate is rapidly exposed to the corrosive environment at the base of the pores. The porosity of IVD aluminium coatings can be reduced by compacting the surface with post-plating glass-bead peening treatments. Electrolytically deposited aluminium coatings are generally more dense than as-plated IVD aluminium and thus the former may afford high levels of protection to steel substrates when exposed to aggressive chloride bearing environments.i6*‘* Recent advances at the University of Salford have enabled aluminium coatings to be obtained using unbalanced magnetron sputtering.” Aluminium coatings deposited using this vacuum technique are dense and non-columnar with significantly improved barrier properties over those produced by conventional ion-plating. Magnetron-sputtering can be used to deposit not only pure metals such as aluminium but also alloy coatings with controlled compositions. A drawback with pure aluminium is that since its corrosion potential is not far removed from that of many steels, it is unable to provide adequate levels of sacrificial protection, particularly under natural exposure conditions.2s22 Earlier work at DRA Farnborough2’ has shown that if aluminium is alloyed with small amounts of active metals such as magnesium, its corrosion potential can be depressed, leading to a significant improvement in sacrificial properties. In the present work, an unbalanced magnetron sputtering technique has been used to deposit Al-Mg alloys with both improved barrier and sacrificial properties. Aluminium alloys containing up to 50% Mg and above have been deposited on to steel substrates and the corrosion behaviour of the sputtered deposits determined using neutral salt fog tests and electrochemical techniques. Particular attention has been paid to determining the effects of magnesium additions on the barrier and sacrificial properties of aluminium.
EXPERIMENTAL
METHODS
The production of coatings Pure Al and Al-Mg alloy coatings were deposited using an unbalanced magnetronsputtering technique at the University of Salford. The use of magnetron-sputtering has been reviewed in several published articles23-25 and precise details of the unit and its operation in the present work are described elsewhere.” In the present work, mild steel test panels of 1 mm thickness (50 x 50 mm dimensions) were initially degreased using an ultrasonic bath containing alcohol and then placed in the vacuum chamber. The magnetron sputtering chamber was evacuated to lo-’ Torr or below and then backfilled with high purity argon to typically 5 x lop3 Torr. The chamber contained two consumable targets, one of commercial grade aluminium (99.4% purity) and one of commercial grade magnesium (99.5% purity). Prior to deposition, the steel test panels were sputter cleaned, typically at an applied voltage of 1 kV for 30 min, to remove any traces of surface contamination. Following this high voltage sputter cleaning process, a low-voltage was applied to the test
Aluminium-magnesium alloys
157
panels, typically - 50 V (DC), to provide an external bias during deposition. Unbalanced magnetron sputtering utilises the plasma in front of the metal targets to self bias the substrates during deposition. In the present work, this was achieved by weakening the magnetic field strengths of the targets and allowing the plasma to escape the confines of the target surface and engulf the substrates. However, due to the large substrate assembly, in relation to the size of the targets, and the fact that the targets are only slightly unbalanced, the coating facility at Salford necessitates the use of an external bias supply during deposition. The targets were energised using an electrical current, to typical power levels of 3 kW for the aluminium target and 1.5 kW for magnesium target. The power levels were varied to obtain the required ratio of Al:Mg in the deposits by producing different sputtering rates. The coating process was continued for a sufficient time to allow the required thickness of deposit to build up on the substrates. After the required thickness had been obtained, the deposition process was halted and the coupons allowed to cool under vacuum to prevent oxidation. A typical deposition rate for the pure Al and Al-Mg alloy coatings was 0.3 pm min-’ , depending on the power supplied to the targets. The composition of each alloy coating was checked using electron-microprobe analysis. The sputtering process was used to produce coatings with thicknesses of 5, 7.5, 10, 15, 25 and 2.5pm. The thickness of each coating was measured prior to testing using a nondestructive eddy-current method. The coatings were not subjected to any post-plating treatments and were tested in the as-plated condition only. Neutral salt fog tests
The corrosion resistance of as-plated pure Al and Al-Mg alloy coatings containing up to 60 wt% Mg were evaluated using neutral salt fog tests conducted in accordance with ASTM Bl 17.26The salt fog cabinet was fed from a reservoir containing 5% w/v NaCl solution with no pH adjustment. The NaCl solution was pumped to an atomizing jet and compressed air directed into the stream of solution to produce a dense salt fog. The test panels were mounted on Perspex racks at an angle of 20” to the vertical and inspected daily for signs of corrosion. Open-circuit
potential
measurements
Open-circuit corrosion potential measurements were conducted using test electrodes constructed from as-plated pure Al and Al-Mg alloy coatings of 25 pm thickness on steel substrates, The test electrodes were separately immersed in 600 mmol 1-l (3.5% w/v) NaCl solution (100 ml) contained in polypropylene beakers. The beakers were positioned in a water bath maintained at 25 f 2°C. A Luggin capillary, in solution contact with an external reference electrode (SCE), was positioned adjacent to the surface of each test electrode and the potential measured at 30 min intervals using a programmable voltmeter. The experiments were halted after 200 h in each case. Polarization
sweeps
Cathodic potentiodynamic polarization sweeps were carried out in a conventional threeelectrode glass cell using test electrodes constructed from sputtered pure Al and Al-Mg alloy coatings of 25 pm thickness. The test electrodes were immersed in 600 mmol 1-l NaCl solution at 25 + 1“C contained in the glass cell. Prior to each sweep, the test electrodes were permitted to stabilise for approximately 30 min. Immediately following the stabilisation period, the electrodes were polarized cathodically at a sweep rate of 0.125 mV SF’. The
158
K.R. Baldwin
sweeps were conducted experiment.
in solutions
et al.
which were gently aerated
EXPERIMENTAL
both before and during
the
RESULTS
Coating microstructure The microstructure of magnetron-sputtered pure Al and Al-Mg alloy coatings was investigated using scanning electron microscopy (SEM). Figure l(a) shows the surface of a pure Al sputter deposit on a steel substrate. The pure Al coating was flat and almost completely featureless which is typical of the dense, non-columnar, coatings which can be deposited by unbalanced magnetron sputtering.” The pure Al coating appeared to afford complete coverage to the substrate and was almost completely free of holidays (small pores) or other microscopic surface defects.
(a) Pm Aluminium
(d) k-57 wt% r&g
(c) Al-32 wt% Mg w Fig.
1. SEM micrographs of fracture cross-sections of unbalanced magnetron and AI-Mg alloy coatings on steel substrates. The black bar at the bottom
sputtered pure Al is 4 pm long.
Aluminium-magnesium
159
alloys
Figures l(b)-l(d) show fracture cross-sections of Al alloy coatings containing 5, 32 and 57 wt% Mg. The Al-Mg alloy coatings were similar to pure Al in that the sputter deposits were flat and generally featureless indicating that the Mg content had very little effect on the microstructure of the coatings. The pure Al and Al-Mg alloy coatings exhibited extremely good adhesion to the steel substrate. Neutral salt fog tests
The corrosion of the sputtered pure Al and Al-Mg alloy deposits containing up to 30 wt% Mg in the salt fog environment was initially characterized by the formation of large pits in the coating surface, although the coatings remained generally bright. White corrosion products, commonly termed white-rust, were found to build-up in the region of the pits and this was usually accompanied by the formation of gelatinous corrosion products. The Al alloy coatings containing over 30 wt% Mg were generally less susceptible to localized pitting attack but instead appeared to corrode in a more uniform manner. With prolonged exposure to the salt fog, the coatings ceased to provide full protection against attack and corrosion of the underlying metal occurred, giving rise to the formation of red-rust, the iron oxide (FezOs) corrosion products of the ferrous substrate. Figure 2 illustrates the time taken for the appearance of first red-rust as a function of alloy Mg content for 8 pm thick coatings. It was found that the time to red-rust increased as the alloy Mg content was increased until a peak in corrosion performance was obtained for Al alloys containing approximately 20 wt% Mg. Further increases in the magnesium content were of no further benefit but instead caused a decrease in the corrosion resistance of the Al-Mg
I
10 Fig. 2. The relationship magnetron sputtered
I
I
I
30 40 20 Ut X Mg alloying addition
I
50
between time to red-rust and alloy composition for 8 pm thick unbalanced AI-Mg alloy coatings on steel-exposed to continuous neutral salt fog.
K.R. Baldwin et al.
160
alloy coatings. Figure 2 shows that the sputtered Aiwt% Mg alloy delayed the onset of red-rust for almost twice as long as the pure aluminium coatings. The effect of coating thickness on time to red-rust for sputtered pure Al and Al alloys containing 5, 10, 15, 30 and 55 wt% magnesium is given in Fig. 3. For pure aluminium coatings, the time to red-rust increased linearly as the thickness was increased, which is in good agreement with earlier work’6,18 on the corrosion behaviour of IVD pure Al coatings in the neutral salt fog environment. Figure 3 shows that, in contrast with the pure Al coatings, for the Al-Mg alloys as the Mg level was increased up to 30 wt%, a significant deviation from linearity occurred in the thickness-time to red-rust relationship. This was particularly pronounced for the Al-30 wt% Mg alloy coatings, where the time to red-rust increased sharply for a relatively small increase in coating thickness. When the concentration of magnesium metal was increased to 55 wt%, a return to linear behaviour was obtained. Information relating to the kinetics of coating breakdown can be obtained by determining the rate at which red-rust forms on the panel surfaces once full barrier and sacrificial protection have been lost. The relationship between the area of red-rust and exposure time for 8pm thick sputtered coatings is shown by Fig. 4. For pure Al and the Al alloys containing up to 35 wt% magnesium, the spread of red-rust was limited, with the area of rust increasingly very slowly for extended exposure periods. Following this initial period, where the coating remained largely intact, the rate of coating loss increased sharply with the
Time
tD red-rust
Ihours)
Fig. 3. The relationship between coating thickness and time to red-rust for unbalanced sputtered pure Al and Al-Mg alloy coatings on steel-exposed to continuous neutral
magnetron salt fog.
Aluminium-magnesium
Exposure
time
alloys
161
(hours)
Fig. 4. The relationship between area of red-rust and exposure time for unbalanced sputtered pure Al and AI-Mg alloy coatings on steel-exposed to continuous neutral
magnetron salt fog.
sputtered deposits being rapidly consumed. It is possible that this type of behaviour was related to the dense nature of the sputtered deposits. The coatings may have become very thin before a significant area of the substrate became exposed to the corrodant so that rapid corrosion of the underlying steel would have only occurred at the moment when the coating was finally lost. Figure 4 shows that the Al-55 wt% Mg alloy coating was consumed rapidly once red-rust had initiated, reflecting the poor corrosion resistance of this high-Mg alloy. Open circuit potential
measurements
The variation in open circuit corrosion potential, EC,,,, with time for sputtered pure Al and the alloys containing 5, 10, 1.5, 25 and 55 wt% Mg, recorded during immersion in quiescent 600 mmol 1-l NaCl solution, is shown by Fig. 5. The test electrode constructed from pure Al initially adopted a potential of - 0.90 V (SCE) in the chloride solution. The potential then increased sharply over the first few hours of immersion to a value of approximately - 0.74 V (SCE) which is similar to that previously recorded for ion-plated aluminium in quiescent sodium chloride solutions.16*18The corrosion behaviour of the pure Al coatings was characterized by frequent potential fluctuations of up to 0.1 V in magnitude, which are usually attributed to the effects of localized corrosion such as pitting attack.27-30 Figure 5 shows that for aluminium alloys containing the lower magnesium contents of 15 wt% and below, similar potential-time behaviour was observed to that obtained for sputtered pure aluminium, with a steep rise in potential being followed by a period of rapid potential fluctuations. In contrast, when the alloy magnesium content was further increased to 25 wt%, the open circuit potential was generally more stable with the fluctuations in potential being less pronounced. This suggests that the Al alloys containing moderate levels of magnesium may be more resistant to pitting attack than pure Al or those containing low levels of Mg, supporting the observations made in the salt fog environment. This trend was continued with the aluminium alloy containing 55 wt% magnesium, where very few potential fluctuations were observed. However, the Al-55 wt% Mg alloy behaved differently from the alloys which were mainly composed of Al in that following a very large rise in open-circuit potential in the first 100 h of immersion, the potential did not stabilise but continued to drift in a noble direction.
er al.
K.R. Baldwin
162
Pure Aluminium
4
(D
-1.01 0
100 Tlme (hours1
Time
fhoursl
v .
Al-5% Mg
m ,
,
,
,
,
,
Time Ihours)
Time lhoursl
jg
-0.6
200
Time lhoursl
Time Ihours)
Fig. 5. The variation in open circuit corrosion potential with time for unbalanced magnetron sputtered pure Al and AI-Mg alloy coatings containing 5, IO. 15, 25 and 55 wt% Mg, determined in quiescent 600 mmol I-’ sodium chloride solution.
The average open-circuit corrosion potential of the sputtered deposits was calculated over the 200 h immersion period in 600 mmol 1-t NaCl solution. Figure 6 illustrates the relationship between the average open-circuit corrosion potential and Al-Mg alloy composition. This plot shows that the average potential of the Al alloys became increasingly negative as the Mg content was increased, which is in line with a previous study on Al-Mg alloys carried out at DRA Farnborough.” The open-circuit corrosion potential of bare steel has been determined in earlier workI and a value of -0.67 V (SCE)
Aluminium-magnesium
iii
-0.6
ti -0.7
a, : -1.0 & z
163
alloys
r___-___-___________-~---
EeOPr tsteell-------
-
-1.1-
I 0
I
I
10
20
WtX Mg
alloying
I
30 addition
I
40
I
50
Fig. 6. The relationship between average open circuit corrosion potential and alloy composition for unbalanced magnetron sputtered AI-Mg alloy coatings, determined over 200 h immersion in 600 mmol 1-l NaCl solution.
was obtained in quiescent 600 mmol 1-l NaCl solution. Figure 6 shows that the sputtered deposits were negative with respect to bare steel and that the Al alloys will become increasingly sacrificial as the concentration of the more active Mg metal is increased. Potentiodynamic
polarization
sweeps
The cathodic polarization sweeps obtained for magnetron-sputtered pure Al and selected Al-Mg alloys in aerated 600 mmol 1-l NaCl solution are shown by Fig. 7. For the pure Al coatings, a rapid increase in current density was initially observed close to the opencircuit potential. The cathodic response then showed highly polarized behaviour, with only a very small increase in current density being observed for a large increase in overpotential. This behaviour is typical of corrosion processes that are under diffusion control, e.g. oxygen reduction.31 Figure 7 shows that the cathodic polarization response of the Al-Mg alloys was similar to that obtained for pure Al in that diffusion controlled cathodic reactions appeared to be predominant. However, it was found that the polarization curves obtained for the Al-Mg alloy coatings were, in most cases, less polarized than pure aluminium, with the alloys showing some deviation from purely diffusion limited behaviour, particularly at the higher overpotentials. A further effect of increasing the alloy magnesium content was to shift the cathodic polarization curves to progressively higher current densities. The corrosion current density was determined for each sputtered deposit using a Tafel extrapolation technique.32 The corrosion current density values obtained are plotted logarithmically as a function of magnesium content by Fig. 8. This plot shows that the corrosion current density of the Al alloys increased as the Mg content was increased and suggests that the barrier corrosion resistance of sputtered Al will be progressively diminished as the concentration of Mg in the alloys is raised.
K.R.
164
Baldwin
et al
-7oc
-900 z 5: : >
M9 .5x M9 -1100
E r( II) .?I JJ s &J -1300 a0
J
-1500 I
-3
-2
Log.Current
-1 Density
0
i
2
$tA/mmz)
Fig. 7. Cathodic potentiodynamic polarization sweeps of unbalanced magneton sputtered pure Al and AI-Mg alloy coatings in aerated 600mmoll-’ sodium chloride solution. Sweep rate: 0.125mVs-‘.
Figure 7 showed that as the alloy magnesium content was increased the cathodic polarization curves were shifted to lower potentials. The relationship between the corrosion current density and the open circuit potentials of the sputtered deposits determined after 1 h immersion in aerated 600 mmol I-‘NaCl solution is shown by Fig. 9. The plot obtained for the Al-Mg alloys indicates that the corrosion current density increased as the open circuit potential became more negative. This type of relationship has been observed for other binary alloy systems33*34 and demonstrates that the corrosion of the Al-Mg alloys was predominantly under cathodic control since the corrosion current became larger as the potential became more negative.
DISCUSSION Barrier properties
of coatings
The cathodic polarization sweeps conducted on sputtered pure Al showed the type of highly polarized behaviour normally associated with corrosion reactions that are under diffusion control (Fig. 7). In the near neutral solutions employed in this study, the predominant cathodic reaction is considered to be oxygen reduction. The cathodic corrosion behaviour of the sputtered Al alloys was similar to that of pure Al even for those alloys containing high Mg levels. For example, the Al-55wt% Mg alloy behaved in a similar manner to pure Al in that diffusion control was predominant, rather than showing
Aluminium-magnesium
alloys
165
0.
Magnesium Fig. 8.
content
(wtX1
The relationship between corrosion current density and magnesium content for unbalanced magnetron sputtered AI-Mg alloy coatings in aerated 600 mmol I-’ NaCl solution.
the more activebehaviour associated with pure magnesium where hydrogen evolution is often the principal cathodic process.3S37 The cathodic polarization sweeps showed that the corrosion current density of pure Al was shifted to higher currents as the Mg level was raised (Figs 7 and 8). If the cathodic current was determined solely by the diffusion limited reduction of oxygen, then no effect of the nature of the cathode should be observed and the corrosion current should be independent of alloy composition. The fact that the currents were very different shows that the nature of the alloy surface was significant. The addition to aluminium of significant concentrations of magnesium would certainly be expected to change the nature of the passive surface film on Al. In the presence of moisture, Mg develops a thin film of magnesium hydroxide, Mg(OH)2, which is generally incomplete and provides only limited protection to magnesium. Thus the addition of sufficient concentrations of Mg to Al would be expected to disrupt the surface oxide film formed on Al and weaken its protective effect, possibly accounting for the poorer barrier properties of the Al-Mg binary alloys. The increase in the cathodic current that occurs as the magnesium content is raised may be related to the increased rate of oxygen diffusion through the less protective oxide film formed on the Al-Mg alloys. Mansfeld et a1.38 employed similar arguments to explain the difference in corrosion rates between aluminium alloys where none should be found since their corrosion reactions were under cathodic control. Mansfeld et al.38 also proposed that differences in efficiency for oxygen reduction on individual Al alloys themselves could also account for the observed difference in their corrosion rates.
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Open-circuit
potential
(V vs SCEl
Fig. 9. The relationship between corrosion current density and open circuit corrosion potential for unbalanced magnetron sputtered AI-Mg alloys determined after 1h immersion in aerated 600 mmol I-’ NaCl solution. Weight percentage magnesium content ofeach alloy shown in brackets.
The barrier corrosion resistance of the Al coatings will not only be dependent on the rate at which they corrode but also the nature of the corrosion itself. In the neutral salt fog environment it was found that the corrosion of the Al and Al-Mg alloy coatings was characterized mainly by pitting attack. This type of localized attack is thought to result from the presence of flaws in the passive oxide film. In studies on pure Al coatings, Hinton et al.” proposed that if the local microstructure exhibited little or no porosity, attack occurred mainly by uniform penetration over a wide area, while some pitting was prevalent at grain boundaries. With as-plated ion-vapour deposited coatings, Hinton et al.” found that the local microstructure contained high levels of porosity, and was thus characterized mainly by localized pitting, leading to early exposure of the substrate. In the present work, the corrosion of the dense, non-porous sputtered Al alloys containing low Mg levels (< 15 wt%) was characterized by the formation of large pits similar in form to those observed for pure Al. The susceptibility of these coatings to pitting attack was demonstrated by the open circuit corrosion potential measurements which showed potential fluctuations which are typical of this type of localized attack. In contrast, those Al-Mg alloys containing higher Mg levels were less susceptible to pitting attack. A possible explanation is that the high Mg alloys were gradually assuming the characteristics of pure Mg for which a more uniform by the open circuit potential type of attack is often found.36337 This was supported measurements which showed a marked increase in electrochemical stability as the alloy Mg level was increased (Fig. 5).
Aluminium-magnesium
alloys
167
For pure Al and the Al-Mg alloy coatings, the air-formed film will provide the first barrier against attack in the presence of an aggressive environment. In the presence of chloride ions, appreciable thinning of the oxide film is likely to occur, and eventually the underlying metallic layer will be exposed to the corrodant. The corrodant will penetrate preexisting defects in the coating such as small pores, which will then behave as crevices. In the pores or in pits formed as a result of corrosion, the local environment will become increasingly acidic. In the case of pure Al, the development of local acidity will accelerate the dissolution of Al and thus stimulate pit growth.39 For the Al-Mg alloys, the development of strongly acidic conditions may lead to the dissolution of both Al and Mg. In the early stages of pit growth, hydrogen evolution would occur because of the limited oxygen supply and the low solubility of oxygen in solutions of high chloride concentration. lo Following this initial period, the pH in the pit is thought to rise as oxygen reduction becomes the predominant cathodic reaction thus generating hydroxyl ions. In the case of pure Al, hydroxides such as Al(OH)s, then precipitate in the pit, accompanied by the formation of other insoluble corrosion products such as Mg(OH)2 in the case of the Al-Mg alloys which may help to stifle corrosion activity. The sacrificialproperties
of coatings
The electrochemical potential of binary alloys is related to the thermodynamic equilibrium potentials of the two components of the alloy.40 The electrochemical potential adopted by the binary alloy will be located between the potential values of the parent metals. The potential of pure magnesium is approximately - 1.9 V (SCE) in 600 mmol 1-l chloride solution35-37 compared with a value of between -0.7 and - 1.OV (SCE) for aluminium, depending upon its purity.‘5-18 Accordingly, the addition of Mg to the sputtered Al deposits produced alloys which adopted open circuit corrosion potentials that were negative to that of pure Al but were positive with respect to that of pure Mg (Fig. 6). Previous work at DRA Farnborough4i has shown that open circuit potential measurements can be a reliable guide in predicting the direction of current flow in a galvanic couple formed between metal coatings and steel although the magnitude of the current cannot be determined solely from potential data and must usually be determined experimentally. If the potential of the coating is negative to steel then sacrificial protection will be obtained whereas if it is more noble than the substrate, the steel will corrode preferentially. In the present work, the open circuit potential adopted by pure Al was close to that of bare steel which, as indicated above, adopts a potential of -0.67 V (SCE) 600 mmol 1-l NaCl solution.16 Although pure Al is slightly negative to steel, it is possible that the coatings will be insufficiently active to drive the potential of the steel substrate well below its protection potential, accounting for the limited sacrificial protection afforded by the pure Al. The sputter deposited Al alloy coatings were found to become increasingly active as the magnesium content was raised, indicating that the coatings would afford improved sacrificial protection to steel over that obtained for pure Al. Figures 5 and 6 showed that alloying additions of 15 wt% Mg and above are required to obtain the significant negative shift in potential to ensure that steel would be effectively protected at any pre-existing pores or corrosion pits in the coating. If the magnesium level was increased significantly above this value, to over 30 wt% Mg, the potential of the alloys was found to decrease below - 1.OV (SCE). Although Al-Mg alloys which adopt such active potentials would provide very effective sacrificial protection, it is likely that they would not be present for long since they are likely to suffer high rates of galvanic corrosion.
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Binary alloy systems composed of two metals with widely different electrochemical potentials are often susceptible to de-alloying. This occurs with, for example, some types of copper-zinc alloys (brasses) where the more active metal (Zn) selectively dissolves from the alloy matrix leaving a deposit enriched in the more noble metal (Cu). In simple terms, the Al alloys containing less than 50 wt% magnesium are analogous to the Cu-Zn alloys in that the more active metal (Mg) is contained in a matrix composed of the more noble metal (Al). An earlier study conducted at DRA Farnborough42 has shown that open circuit potential monitoring is useful in studying de-alloying since a significant positive shift in potential generally occurs as the more active component of the alloy is dissolved from the alloy surface. In the present work, the potential of the Al alloys containing less than 50 wt% Mg remained relatively stable despite the fluctuations in potential, suggesting that the Al-Mg alloys which were composed mainly of Al were resistant to de-alloying. The open circuit corrosion potential measurements conducted on the sputtered alloys composed mainly of Mg were found to behave differently to those composed mainly of Al. Fig. 5 shows that the potential of the Al-55 wt% Mg alloy increased sharply over the initial immersion period. In contrast to pure Al and the low-magnesium alloys which also showed initial rises in potential, the Al-55 wt% Mg alloy became increasingly noble and did not stabilise suggesting that it is susceptible to a form of de-alloying. A possible explanation is that magnesium was selectively dissolved leaving a surface enriched in the more noble aluminium, accounting for the noble shift in potential. Toward the end of the immersion period, the potential of the high-magnesium alloy did eventually show signs of stabilising, possibly as an equilibrium structure developed in the surface layers. Corrosion behaviour iiz neutral salt f5g In the neutral salt fog tests, it was found that the level of corrosion protection afforded to steel by the sputtered Al coatings increased sharply as the magnesium content was increased until a peak in performance was observed for alloys containing 20 wt% magnesium. Further increases in the magnesium content were of no further benefit and instead caused a decline in corrosion performance. The cathodic polarization sweeps showed that the corrosion current density of Al in 600 mmol l- ’ NaCl solution was increased as the Mg level was raised. The values obtained for Al alloys with Mg concentrations of up to approximately 20 wt% were lower than corrosion currents previously recorded for ion-vapour deposited pure Al,’ where the corrosion current density of the as-plated IVD coatings in aerated 600 mmol 1-l was found to be approximately 0.01 PA mm-‘, which is 10 times higher than that recorded in the present work for sputtered pure Al under very similar experimental conditions. The significantly higher corrosion current density found for the ion-vapour deposited aluminium coating is most likely due to its high porosity compared with the sputtered deposits evaluated in this study. Thus it may be concluded that although the addition of significant concentrations of Mg would have reduced the barrier corrosion protection afforded by the Al alloy sputtered deposits, they would remain more effective than conventional ion-plated aluminium coatings provided the Mg content does not exceed approximately 20 wt %. In the neutral salt fog environment, it is proposed that the initial increase in the level of corrosion protection observed in the neutral salt fog environment for alloys containing up to 20 wt% magnesium was mainly due to an improvement in the level of sacrificial protection afforded by the sputtered deposits as the Mg concentration was increased. It has been shown that the loss of barrier properties would not have become a significant factor until the
Aluminium-magnesium
alloys
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magnesium content was relatively high ( > 20 wt% Mg). The decline in the level of corrosion protection afforded by the sputtered Al-Mg alloys (Fig. 2) when the magnesium level was increased above 20 wt% was most likely due to the combined effects of excessive rates of galvanic corrosion as the coatings became more active and the loss of barrier corrosion protection. Thus the data obtained in the present work suggest that the peak in performance observed in the neutral salt fog environment for the sputtered deposits containing approximately 20 wt% Mg represents an optimum balance being attained between the barrier and sacrificial properties of the Al-Mg alloy coatings. CONCLUSIONS 1. Potentiodynamic cathodic polarization sweeps conducted on unbalanced magnetron sputtered Al-Mg alloy coatings in 600mmol I-’ NaCl solution showed that the corrosion current density of the Al alloys increased as the Mg content was raised, indicating that the barrier corrosion resistance of sputtered Al will decline as the alloy Mg level is increased. 2. From open circuit corrosion potential measurements conducted in 600 mmol l- ’ NaCl solution it was found that sputter deposited Al alloy coatings become progressively more active as the alloy Mg content is increased, suggesting that the Al alloys will become more sacrificial as the Mg level is raised. 3. In neutral salt fog tests, the level of corrosion protection afforded by Al alloy coatings is improved by the addition of Mg until an optimum is reached for alloys containing approximately 20 wt% Mg. Further increases in the Mg level, above 20 wt%, caused a decline in performance. It is proposed that the peak in performance represents an optimum balance attained in the neutral salt fog environment between the barrier and sacrificial properties of the Al-Mg alloy coatings. REFERENCES I G.A. Capuano, R. Lafortune and C. Gobeil, Plating and Surface Finishing &I,49 (1977). 2. R. Suchentrunk, Corrosion Protection by Electra-deposited Aluminium, AGARD Lecture Series 106, I. l-7.25 (1980). 3. W. Wittich. R. Suchentrunk and H. Kellerer, Metal1 und Technik 30, 943 (1980). 4. M. Galova, Surf. Technol. 11, 357 (1980). 5. S. Biallozor. A. Lisowska-Oleksiak and W.N. Titova, Surf Coat. Technol. 34, 549 (1988). 6. W. Kautek, Corros. Sci. 28, I73 (1988). 7. E. Fanin, AGARD Conf Proc., No. 256, Advanced Fabrication Processes, Florence, Italy, 2628 September (1978). 8. M. El-Sherbiny and E. Salem, Anti-corrosion 28, 15 (1981). 9. N.A.G. Ahmed, Anti-Corrosion 31, 4 (1984). 10. B.R.W. Hinton, W.J. Pollock, P.N. Trathen and A. Crosky, Corros. Australasia 13, 12 (1987). 11. R. Palmer and D.J. Hall, Product Finishing 45, 6 (1992). 12. B.T. Nevill, Plating Surf Finishing 80, 14 (1993). 13. J.F. McIntyre and L. Hang, Corrosion 50, 26 (1994). 14. J.C. Salvador Fernandes and M.G.S. Ferreira, Surf Coat. Technol. 53, 99 (1992). 15. W.H. Ailor, Proc. ConfI Corrosion ‘70, Paper No. 36, p. 200. Philadelphia (1970). 16. C.J.E. Smith and K.R. Baldwin, Product Finishing 45, 12 (1992). 17. T.D. Burleigh, R.C. Rennick and F.S. Bovard, Corrosion 49, 683 (1993). 18. K.R. Baldwin and C.J.E. Smith, Proc. Co@ Materials Technology ‘91 (ed. A. Niku-Lari), p. 153. IITTTechnology Transfer International, Paris, September (1991).
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19. D.P. Monaghan. D.G. Teer, P.A. Logan, K.C. Laing, R.I. Bates and R.D. Amell, Surf. Coat. Technol. 60, 592 (1993). 20. P.L. Lane and C.J.E. Smith, Surface Engineering Practise (ed. K.N. Strafford, P.K. Datta and J.S. Gray), p. 566. Ellis-Horwood Ltd., Chichester (1990). 21. J.B. Horton, Corrosion Control by Coatings (ed. H. Leidheiser), Science Press, p. 59. Princeton (1979). 22. V.C.R. McLaughlin. AGARD Cant Proc.. No. 256, Advanced Fabrication Processes, Florence, Italy, 2628 September (1978). 23. B. Window and N. Savvides, J. Var. Sri. Technol. A4, 196 (1986). 24. B. Window and N. Savvides, J. Var. Sci. Technol. A4, 453 (1986). 25. B. Window and N. Savvides, J. Var. Sri. Technol. A4, 504 (1986). 26. ASTM B117. Standard Method of Salt Spray (Fog) Testing, American Society for Testing and Materials, Philadelphia (1981). 27. M. Janik-Czachor, G.C. Wood and G.E. Thompson, Br. Corros. J. 15, 154 (1980). 28. B. Samuels, K. Sotoudeh and R.T. Foley, Corrosion 37, 92 (1981). 29. R.T. Foley, Corrosion 42, 277 (1986). 30. K. Nisancioglu and H. Holtan, Werkstoffe und Korrosion 30, 105 (1979). 31. M.G. Fontana and N.D. Greene, Corros. Eng. Ch. 9. McGraw-Hill Book Co., New York (1967). 32. F. Mansfeld, Electrochemical Techniques for Corrosion (ed. R. Baboian), p. 18. NACE (1977). 33. K.R. Baldwin, M.J. Robinson and C.J.E. Smith, Corros. Sri. 36, 1115 (1994). 34. N.R. Short, A. Abibsi and J.K. Dennis, Trans. Inst. Met. Finishing 67, 73 (1991). 35. F. Hehmann, F. Sommer, H. Jones and R.G.J. Edyvean, J. Mar. Sci. 24, 2369 (1989). 36. G.L. Makar and J. Kruger, J. electrochem. Sot. 137, 414 (1990). 37. G.L. Makar, J. Kruger and K. Sieradzki. J. electrochem. Sot. 139, 47 (1992). 38. F. Mansfeld, D.H. Hengstenberg and J.V. Kenkel, Corrosion 30, 343 (1974). 39. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Section 5.1. 2nd Edition. NACE; Cebelcor (1974). 40. H. Kaiser, Corrosion Mechanisms (ed. F. Mansfeld), p. 85. Marcel Dekker Inc. (1987). 41. K.R. Baldwin, M.J. Robinson and C.J.E. Smith. Br. Corrosion J. 29, 299 (1994). 42. K.R. Baldwin, M.J. Robinson and C.J.E. Smith. Corros. Sci. 35, 1267 (1993).