Galvanic Interactions of Aluminium 3004 and ∝ Brass in Tropical Marine Atmosphere

S. Palraj, G. Subramanianand S. Palanichamy

1. CSIR-Central Electrochemical Research Institute, Karaikudi-630006, India

2. Offshore Platform & Marine Electrochemistry Centre, CSIR-CECRI Unit, Harbour Area, Tuticorin-628004, India

Galvanic Interactions of Aluminium 3004 and ∝ Brass in Tropical Marine Atmosphere

S. Palraj1, G. Subramanian2＊and S. Palanichamy2

1. CSIR-Central Electrochemical Research Institute, Karaikudi-630006, India

2. Offshore Platform & Marine Electrochemistry Centre, CSIR-CECRI Unit, Harbour Area, Tuticorin-628004, India

The galvanic corrosion behaviour of aluminium 3004 - ∝brass with different area ratios was studied in the tropical marine atmosphere at Tuticorin harbour over a period of 426 days. The area ratios, viz. AAluminium:A∝brass, studied were 0.125, 0.25, 0.5, 1, 2, 4 and 8. The galvanic corrosion behaviour of the metals was studied in terms of the relative increase in the corrosion rate of aluminium due to galvanic coupling with ∝ brass, the relative decrease in the corrosion rate of ∝ brass due to galvanic coupling with aluminium, and the susceptibility of aluminium to pitting owing to galvanic coupling with ∝ brass. The galvanic potential and galvanic current of the system were monitored. Pits of different dimensions ranging from mild etchings to perforations were experienced on the borders and the surfaces of the interface of aluminium in contact with ∝ brass. The corrosion products resulting from galvanic corrosion were analysed using XRD and the pitting on aluminium as a result of galvanic corrosion was highlighted in terms of pit depth, size and density of pit, using a high resolution microscope. The most favourable area ratio of aluminium - ∝ brass in marine atmosphere in terms of gravimetric corrosion rate is 8:1 and the most unfavourable area ratio of aluminium - ∝ brass is 1:4.

aluminium 3004; ∝ brass; galvanic corrosion; corrosion products; pitting; tropical marine atmosphere

1 Introduction

Galvanic corrosion occurs commonly in atmospheric environments as different combinations of materials are used in buildings and structures exposed to indoor and outdoor atmospheres. Galvanic corrosion is caused by an electrolytic action, typified when naturally occurring low electrical current (telluric currents/voltage) and the presence of moisture are combined with any dissimilar metals. Coastal regions with sea spray, mist and high humidity are more susceptible to corrosion due to sodium chloride (salt/saline) present in coastal atmosphere which attacks metals and paint finishes (Phull and Kain, 2004). The use of dissimilar metals in structural design is fairly common, particularly cases where the fastener material is different from the structure being joined. Military materials are deployed and used worldwide under varied and severe environmental conditions. In the design and fabrication of equipment the use of electrochemically similar metal is desirable, but is not feasible in many instances when structural, electrical and other important design criteria are taken into account. Consequently under such conditions, especially in marine atmospheres, significant galvanic corrosion problems are encountered. Although the danger of galvanic corrosion has been recognized generally, very little work has been done to quantitatively measure the extent of corrosion of various galvanic couples, and use of such data as the basis for ranking of galvanic couples (Subramanian, et al., 1999, 2014). The magnitude of galvanic corrosion depends not only on the potential difference of dissimilar metals, but also on the kinetic parameters such as corrosion rates and area ratios (Mansfeld, 1971; LaQue, 1951). Though measurements of electrode potential indicate the protective nature of the corrosion products (Vassie and Mckenzie, 1985), firsthand knowledge on galvanic current and weight-loss data, throw more light on the galvanic interactions of the metals in contact (Mansfeld and Kenkel, 1975). Pitting is the form of localized corrosion attack usually encountered, when dissimilar metals are in galvanic contact. By virtue of environmental pollutants and weathering conditions (Campbell, 1950), this can lead to premature failure of the systems in operation.

Aluminium is used excessively in the modern world, and the uses of the metal are extremely diverse due to its many unusual combinations of properties. Applications for aluminium include ship hulls, piers, tank interiors, offshore structure, submerged pipelines, piling (Markley et al., 2007). Copper alloys are very widely used material for its excellent electrical and thermal conductivities in many industrial applications (Barouni et al., 2008; Musa et al., 2011a, 2011b).

Brass is the best material to manufacture many components because of its unique combinations of properties. Good strength and ductility are combined with excellent corrosion resistance and superb machinability. Brasses set the standard by which the machinability of other materials is judged and are also available in a very wide variety of product forms and sizes to allow minimummachining to finished dimensions. Among the copper based alloys, the brasses have excellent resistance to normal atmospheric corrosion and this is one of the key properties in materials selection decisions. For this reason brass is the first choice material to give many years of satisfactory service for many common but critical applications such as electrical components, scientific and other accurate instruments, clocks, hose and pipe fittings, etc. The high tensile strength’ alpha-beta brasses which can be cast, hot-rolled, forged or extruded; applications include propellers and marine hardware including shackles. Typical applications for marine environments include heat exchangers and condensers, seawater piping, hydraulic tubing, pump and valve components, bearings, fasteners, marine fittings, propellers, shafts, offshore sheathing and aquaculture cages. In outdoor exposure conditions, especially where there is industrial pollution of the atmosphere or in situations very close to the sea, a heavier tarnish develops on most brasses. This eventually produces a thin deposit of brown-green copper compounds which is adherent and spreads uniformly across the surface, helps to protect against further attack (Webster, 2005).

A test program of galvanic corrosion in atmospheres was started as early as 1931 by the American Society for Testing and Materials and the various aspects of atmospheric galvanic corrosion have been discussed in a comprehensive review by Kucera and Mattsson (1982). Since then, a number of extensive exposure programs have been carried out all over the world (Rosenfeld, 1962; Doyle and Wright, 1988; Compton and Mendizza, 1955; Dey et al., 1966). However in tropical countries like India, the galvanic corrosion studies of various metals and alloys in marine atmosphere are totally lacking.

Better understanding on the corrosion process will help in reducing the safety and economic impact of corrosion damage. There has been a need for the specific data relating to galvanic interactions of certain metal combinations frequently encountered in military equipment in tropical marine atmosphere to aid the design engineer in the selection of metal combinations to achieve maximum corrosion resistance.

Hence in the present study, galvanic interactions of aluminium and ∝ brass have been studied in depth to delineate weight-loss corrosion rate, with respect to varying area ratios, galvanic potential, galvanic current, pitting and corrosion products formed at the interface of bimetallic contact, in the tropical marine atmosphere at Tuticorin harbour, Tamilnadu, India.

2 Materials and methods

Commercially available metal sheets such as aluminium 3004 (Mn: 1.05%, Mg: 1.12%, Al: balance) of 1.6 mm thickness and ∝ brass (Cu: 70% and Zn: 30%) of 3 mm thickness supplied by The Lawrence Metal Industries, Chennai were used in this study.

2.1 Coupon preparation

The sheets of aluminium 3004 and ∝ brass were cut into the required number of coupons of the following sizes, 38 mm × 25 mm, 50 mm × 38 mm, 75 mm × 50 mm and 150 mm × 100 mm. All the coupons were provided with a central hole of size 10 mm diameter for effecting galvanic contact. The coupons were polished (ASTM standards, 1991), degreased and weighed to an accuracy of 10-4g. Separate set of coupons was prepared for measuring galvanic current of aluminium 3004 - ∝ brass couples as described elsewhere (Mansfeld and Kenkel, 1976).

2.2 Test site

The offshore platform of CECRI’s centre at Tuticorin harbour is located (longitude 78.13° East and latitude 8.8° North) on the southeast coast of India in the Gulf of Mannar. This platform stands on piles in the open sea at a water depth of 6–7 m, which is about 1.5 km away from the shore. The standard atmospheric corrosion testing racks erected at the offshore platform were inclined 45° to horizontal. Though the test site is influenced by both southwest and northeast monsoon, relatively higher amount of salt content in the atmosphere is recorded during southwest monsoon. The site is characterized by the prevalence of 63%–77% of relative humidity throughout the year.

2.3 Experimental techniques

The area ratios of the couple aluminium 3004 - ∝ brass, viz. AAluminium:A∝brass, studied were 1:1, 1:2, 1:4, 1:8, 2:1, 4:1 and 8:1. Galvanic coupling was effected by placing one over the other (aluminium over ∝ brass) using insulated G.I bolt & nut to avoid tri-metallic contact. The contacts in all couples were made possible with the help of G.I bolt & nut to an extent of equal magnitude. Required number of coupons of freely corroding and galvanic couples in triplicate were exposed by mounting on the atmospheric corrosion test racks. In all the couples, the aluminium coupon was positioned on the top of the ∝ brass coupon to avoid contamination of aluminium by the corrosion products of ∝ brass (Subramanian et al., 1999). The galvanic potential values of the couples were monitored for the area ratios 1:1, 4:1, 1:8 and 8:1, following the technique as described elsewhere (Vassie and Mckenzie, 1985), using high impedance digital multi-meter with respect to saturated calomel electrode (SCE). The galvanic current values of the couples with the area ratios 1:1, 4:1, 1:8 and 8:1, were monitored using the atmospheric corrosion monitor as described elsewhere (Mansfeld and Kenkel, 1976). At no time, the galvanic potential & galvanic current values of the area ratios 1:2, 1:4 and 2:1, were monitored. The test coupons were removed at the intervals of 144, 214, 302 and 426 days. The gravimetric corrosion rates were calculated after removal of corrosion products using the recommended pickling solutions (ASTM standards, 2003). Weight-loss method was used to calculate the corrosion rate (rwl) of metals under galvanically coupled condition, the relative increase in the corrosion rates {(rwl?r0)/r0} of aluminiumdue to galvanic coupling with ∝ brass and the relative decrease in the corrosion rates {(r0?rwl)/r0} of ∝ brass due to galvanic coupling with aluminium, and (r0) the corrosion rate of freely corroding individual metals. The corrosion products were analyzed with X-ray diffractometer (Model-PW3040/60 X’pert PRO). The pitting corrosion behaviour of the galvanically coupled aluminium coupons exposed for 144, 214, 302 and 426 days were recorded in terms of depth, density and size of pits (ASTM standards, 2005), using a high resolution microscope. The pitting factors of the galvanically coupled aluminium coupons of 426 days of exposure were also calculated. The salt content in the air and the meteorological data such as % relative humidity, maximum & minimum temperature, total sunshine hour and rainfall, were recorded periodically over the study period. The salt content in the air was determined by wet candle method, the % relative humidity was monitored using wet & dry bulb thermometers, the maximum & minimum temperature was monitored using maximum minimum temperature system, the rainfall was measured using rain gauge and the sunshine hour was recorded using a calibrated Campbell-Stokes pattern sunshine recorder.

3 Results

Table 1 shows the meteorological data of the test site. The percent relative humidity of the test site varied from 63.5 to 77.4 and the salt content in the air was found to vary from 0.42 to 6.48 mdd. Maximum rainfall (172.3–182.5 mm) was recorded during northeast monsoon period (October–December) and intermittent rainfall (2.5–12.9 mm) was recorded from May to September. Maximum sunshine hours (208.8–298.4 h) were recorded between January & March. Fig. 1 and Fig. 2 illustrate the corrosion rates of freely corroding aluminium & aluminium coupled to ∝ brass and corrosion rates of freely corroding ∝ brass & ∝ brass coupled to aluminium, respectively. In general, the corrosion rates of aluminium coupled to ∝ brass over a wide range of area ratios are higher than that of the freely corroding aluminium, while the corrosion rates of ∝ brass coupled to aluminium over a wide range of area ratios are lower than that of freely corroding ∝ brass, excepting the area ratio of 1:8 for 144 days exposure.

Fig. 1 Corrosion rates of freely corroding aluminium & aluminium coupled to ∝ brass

Table 2 summarizes the data on relative increase/ decrease in the corrosion rate of dissimilar metals of aluminium - ∝ brass. The relative increase in the corrosion rate of aluminium resulting from galvanic coupling with ∝brass was in the descending order of 1:4 ＞ 1:2 ＞ 1:1 ＞ 1:8 ＞2:1 ＞ 4:1 ＞ 8:1 for 144 days of exposure and 1:4 ＞ 1:1 ＞1:2 ＞ 2:1 ＞ 1:8 ＞ 4:1 ＞ 8:1 for 426 days of exposure. The relative decrease in the corrosion rate of ∝ brass in galvanic contact with aluminium was in the descending order of 4:1 ＞ 8:1 ＞ 1:1 ＞ 2:1 ＞ 1:2 ＞ 1:4 ＞ 1:8 for 144 days of exposure and 8:1 ＞ 1:1 = 4:1 ＞ 2:1 ＞ 1:4 ＞ 1:8 ＞ 1:2 for 426 days of exposure. The galvanic potential and galvanic current density values of aluminium - ∝ brass couples (1:1, 4:1, 1:8 and 8:1) over the period of exposures are shown in Fig. 3 and Fig. 4, respectively. The OCP values of control aluminium showed more fluctuations up to 112 days and thereafter attained plateau potential till the end of the experiment. The OCP values of control ∝ brass showed negative shift (?385 mV) from the OCP up to 50 days and thereafter a gradual increase towards nobler direction (?102mV) was observed till the end of the experiment, indicating the protective nature of the corrosion products formed over the period of time. The galvanic potential values didn’t show much variation among the area ratios 1:1, 4:1 and 8:1 and the values were found to be rather on the negative direction as compared to the OCP values of control ∝ brass. However the galvanic potential values of 1:8 area ratio were on the nobler direction when compared to all other area ratios and the OCP values of control ∝brass. Though the galvanic current density values of the 4:1 area ratio were markedly higher when compared to all other area ratios, initially there was a decrease in the values till 85 days and thereafter it tended to increase gradually. No marked variations in the galvanic current density values were observed throughout the study period for the galvanic couples of the area ratios 1:8 and 8:1. However for the area ratio 1:1, there was a slight increase in the galvanic current density values after 200 days, when compared to those values of 1:8 and 8:1.

Fig. 2 Corrosion rates of freely corroding ∝ brass & ∝ brass coupled to aluminium

Fig. 3 Galvanic potential values of aluminium - ∝ brass couples

Fig. 4 Galvanic current density values of aluminium - ∝brass couples

Table 1 Meteorological data and salt content in the air at the test site

Table 3 Pitting on aluminium 3004 resulting from galvanic corrosion of aluminium 3004 - ∝ brass

Table 3 summarizes the pitting on aluminium 3004 in galvanic contact with ∝ brass. Pitting on aluminium coupled to ∝ brass was observed beneath the white corrosion products on the borders and within the interface of the bimetallic contact. The depth of pit varies from 400–600 μm on the surfaces of galvanically corroding aluminium of 1:1, 1:2 and 2:1 area ratios, while the other area ratios & freely corroding aluminium ranges from 200–400 μm, for 144 days. Significant variations in the density and size of pit between the galvanically corroding aluminium (2.5×103/m2, 0.5–26.5 mm2) and freely corroding aluminium (max. No. 2.5×103/m2, 0.6–1.8 mm2) were observed for 144 days of exposure. On the other hand, the depth of pit on galvanically corroding aluminium ranges from 400–1 600 μm for 426 days and that of freely corroding aluminium is limited to a maximum of 400 μm. Significant variations in the density and size of pit between the surfaces of galvanically corroding aluminium (2.5×103–5.0×104/m2, 0.25–39.5 mm2) and freely corroding aluminium (max. No. 2.5×103/m2, 0.5–2.0 mm2) were observed for 426 days of exposure. In general, perforations were experienced by the aluminium coupled to ∝ brass of 1:1, 1:2 and 2:1 area ratios of 144 days and 214 days of exposure. In 426 days of exposure, all the area ratios of aluminium experienced perforations, excepting the 1:8 area ratio. Other than de-zincification and tarnishing, no characteristic pitting is observed on the surface of brass coupled to aluminium.

4 Discussion

The corrosiveness of a marine environment depends on the topography of the shore, wave action at the surf line, prevailing winds and relative humidity. Salt is deposited on materials surfaces by marine fog and windblown spray droplets (deposition rate of NaCl higher than 15 mg·m-2·d-1). This contamination induces severe corrosion at relative humidity exceeding about 55%. This environment is characterized by proximity to the ocean and salt-laden air that can produce very severe corrosion damage on many structural materials, enhance galvanic corrosion, and accelerate deterioration of protective coating systems. Marine atmospheres are usually highly corrosive. The principal culprit in marine atmospheres is the chloride (Clˉ) ion derived from sodium chloride (Syed, 2006).

The surface area of the cathodic members exposed to marine environment normally limits the galvanic current. Therefore, a small cathodic area in contact with a large anodic area can have little effect on the overall corrosion rate of the less noble, anodic material. Alternatively, if the relative area of the more noble, cathodic area is high, then excessively high corrosion rates of the anode might be experienced (Powell and Webster, 2012). In the present study the corrosion rate of galvanically coupled aluminium with ∝ brass was in general, higher than that of the freely corroding aluminium. The corrosion rate of aluminium depends on the area of the ∝ brass, i.e. the corrosion rate of aluminium increases with an increase in area of ∝ brass and vice versa. However in 1:8 area ratio, the corrosion rate of aluminium was found to be decreased throughout the study period, while the corrosion rate of ∝ brass was highest of all the area ratios studied for 144 days of exposure. This may be attributed to the polarity reversal of aluminium - ∝ brass couple. This observation supports the earlier studies doneelsewhere (Schikorr, 1939). The normal polarity of some galvanic couples under certain conditions may reverse with the passage of time. This phenomenon was first reported by Schikorr in 1939 on a zinc-steel couple in hot supply water with iron becoming anodic to zinc, which has been a serious problem for galvanized steel hot water tanks. It has subsequently been extensively investigated (Glass and Ashworth, 1985; Gilbert, 1948; von Fraunhofer and Lubinski, 1974).

Polarity reversal is invariably caused by the change of surface condition of at least one of the coupled metals, such as formation of a passive film. The degree of passivity, the nature of the redox couples in the solution, and the stability of the system determine the polarity and its variation with time (Hoxeng, 1950). Depending on the conditions, it may occur rather quickly, taking several minutes, or rather slowly, taking many days. In addition to temperature, other factors, such as dissolved ions, pH, and time of exposure, affect the polarity of a zinc-steel couple. Polarity reversal of an aluminium-steel couple has also been found to occur in natural environments where aluminium alloys are used as anodes for cathodic protection of steel. The general mechanism is similar to that occurring with a zinc-steel couple. However, unlike zinc, aluminium is normally passivated by a thin oxide film in most natural environments. The potential of aluminium depends on the degree of passivity, which is sensitive to the ionic species in the environment. For example, carbonate and bicarbonate ions promote passivity and, thus, produce more noble potential values, whereas ions like chloride give the opposite effect (Gabe and El Hassan, 1986).

According to Kucera and Mattsson (1982) galvanic action is most significant in marine atmosphere because of the high conductivity of seawater. Compared to other types of moisture formed under atmospheric conditions, rain is particularly effective in causing galvanic corrosion. In the present study, the galvanic corrosion is more severe during the initial exposure period of 144 days. This could be explained that the exposure was commenced during June. The study location usually experiences severe monsoonal wind (southwest) between June & September, which results in higher salt content in the atmosphere, and the intermittent showers of rain during this period resulted in the higher corrosion of metals.

The initial fluctuations in the OCP values of control aluminium may be due to the aggressiveness of the environmental conditions prevailed during that period and the plateau potential is indicative of the protective nature of the corrosion products formed over the period of time. This is further supported by the XRD analysis showing the presence of corrosion products, like aluminium chloride hexa hydrate, Ψ–aluminium oxide, aluminium oxide hydroxide, ∝–aluminium hydroxide. The OCP values of control ∝ brass showed negative shift (?385 mV) from the OCP up to 50 days and thereafter a gradual increase towards nobler direction (?102 mV) was observed till the end of the experiment, indicating the protective nature of the corrosion products formed over the period of time. This is further supported by the XRD analysis showing the presence of corrosion products, like copper (II) chloride di-hydrate & copper (I) chloride, basic zinc chloride, Zn5Cl12(OH)8·H2O, Zn12(OH)15Cl3(SO4)3·5H2O.

In the present study, generally, pitting on aluminium coupled to ∝ brass was observed beneath the white corrosion products on the borders and within the interface of the bimetallic contact. In general, galvanic corrosion in atmosphere is usually restricted to a narrow region of the anode metal near the bimetallic junction because of the high resistance of thin-layer electrolytes formed by rain and water condensation (Rosenfeld, 1962; Zhang, 1998; Zhang and Valeriote, 1995). The extent of pitting experienced by the aluminium coupled to ∝ brass of 1:1, 1:2 and 2:1 area ratios for 144 and 214 days exposure and for all the area ratios excepting 1:8 area ratio for 426 days exposure, is reflected by the aggressiveness of the tropical marine atmosphere, prevailed during that period. The pitting factors for 426 days exposure of galvanically corroding aluminium for the area ratios, viz. 1:1, 1:2, 1:4, 1:8, 2:1, 4:1 and 8:1 are 2.8, 2.4, 3.2, 2.0, 2.2, 2.2 and 2.4, respectively and for the freely corroding aluminium is 0.8. Thus the durability of aluminium is brought down by a factor of 2.5 to 4 due to galvanic coupling with ∝ brass. The pitting factor observed for galvanically coupled aluminium - ∝ brass is higher than that of the galvanically coupled aluminium - copper for the same study location (Subramanian et al., 2014).

5 Conclusions

The most favourable area ratio of aluminium - ∝ brass in marine atmosphere in terms of gravimetric corrosion rate is 8:1 and the most unfavourable area ratio of aluminium - ∝brass is 1:4. The durability of aluminium is brought down by a factor of 2.5 to 4 due to galvanic coupling with ∝ brass. The pitting factor observed for galvanically coupled aluminium of aluminium - ∝ brass is higher than that of the aluminium of galvanically coupled aluminium-copper.

Acknowledgements

The authors are thankful to the Director, CSIR-CECRI, Karaikudi for encouragement. The enthusiastic technical service rendered by Mr. P. Ganapathy, Senior Technical Officer, CECRI Unit, Tuticorin is thankfully acknowledged.

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Author biographies

S. Palraj is currently holding the post of Principal Technical officer at CSIR-CECRI, Karaikudi. He has a PhD in Chemistry and has been working in the field of Corrosion Testing & Evaluation for the last 31 years. Presently he is involved in the development of antifouling coatings. Besides well cited publications, he has eight Indian patents and one US patent to his credit. Further he has developed 4 processes and commercialized them.

G. Subramanian is currently holding the post of Senior Principal Scientist and Head of OPMEC, CECRI Unit, Tuticorin. He has a PhD in Marine Sciences and his research since 1983 has focused on a range of topics including atmospheric corrosion, evaluation of paints and coatings, marine biofouling prevention and corrosion in seawater. Besides well cited publications, he has seven Indian patents and one US patent to his credit. He has also commercialized a process.

S. Palanichamy, senior scientist, holds a PhD degree in marine sciences at CSIR-CECRI, Tuticorin. He was instrumental in establishing a strong database on chemical oceanographic features of coastal ocean waters, including the Tuticorin and Mandapam regions. Simultaneously he also investigated the effects of water chemistry and local pollution on corrosion and biofouling phenomena. Currently he is developing antifouling formulations from marine natural products.

1671-9433(2014)04-0455-07

J. Marine Sci. Appl. (2014) 13: 455-461

10.1007/s11804-014-1274-6

date: 2014-02-14.

Accepted date: 2014-04-11.

Supported by the CSIR-CECRI under project No. MLP 0008.

＊Corresponding author Email: cgscorr@yahoo.co.in

? Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2014