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The effect of aluminium alloying on the corrosion resistance of cast steel

Robert JEFFREY1 & Robert E MELCHERS1 1Centre for Infrastructure Performance and Reliability, The University of Newcastle,

Australia, Robert.Jeffrey@newcastle.edu.au, Rob.Melchers@newcastle.edu.au

Abstract: There is some evidence that the addition of small amount of aluminium can reduce the atmospheric corrosion of steel. However, systematic long-term investigations are lacking. There also appears to be no information about resistance of such alloys to marine immersion conditions. This paper reports on the marine environment corrosion performance of steel alloyed with aluminium additions of 0% (control), 2%, 4%, 6% and 8%. The cast steel was forged into billets and machined into coupons. Sets of coupons were exposed to three exposure conditions: (a) in temperate seawater for 1, 2 and 3 years, (b) submerged below the mud-line for 1, 2 and 3 years and (c) exposed at two extremely severe atmospheric marine locations for 12 months. The results show that the increase in Al content reduced corrosion loss in submerged conditions from 10.5% after one year to 2.8% after 3 years. In the coastal atmosphere, corrosion loss dropped from 5.5% after one year with no Al addition to 1.2% with 8% Al alloying. The corrosion resistance in marine mud buried conditions was less favourable. Corrosion loss reduced from 4.4% after one year to 2.9% after 2 years but in some instance the addition of Al had minimal or no effect on corrosion resistance. These observations add to the body of knowledge regarding the effect of aluminium on long-term corrosion loss.

Keywords: Cast steel, aluminium alloy, atmospheric corrosion, marine corrosion

Introduction

It has been long known that the alloying of steel can alter its physical as well as its chemical properties. Usually increased strength while maintaining ductility at an acceptable level has been the main aim [1]. Alloys also have been added to attempt to reduce corrosion. For example weathering steels typically have additions of 0.40 – 1.25% chromium, 0.25- 0.55% copper, 0.40 – 0.65% nickel and in some grades 0.02 – 0.10% vanadium [2]. There is some evidence that a small amount of aluminium can reduce the atmospheric corrosion of steel but systematic investigations are lacking. Also, there is no information about resistance of such alloys for immersion conditions. Nishimura et al. [3] and Nishimura and Kodama [4] showed that for Fe with 0.8% (mass) Al, after 20 cycles in a wet/dry cyclic corrosion test using a 0.5% (mass) NaCl solution, corrosion was some 75% of the corrosion for mild steel. They also found that the Al was concentrated mainly in the inner rust layers, next to the steel, with magnetite the predominant phase. However, no Al3Fe was found and it was concluded that the Al was present in the spinel-structured magnetite. It was suggested that the presence of Al in that spinel was the likely cause of the increased protective effect of the magnetite against species diffusion, known to be the rate-limiting step in the corrosion process [5]. For ultrafine-grained weathering steels the influence of Al (and Si) on increased corrosion resistance has been attributed to their role in converting complex oxides into fine structure and that these were influential in retarding the diffusion of aggressive species [6]. Electrochemical monitoring of the rusts of low alloy steels with 0.8% (mass) Al alloying exposed to alternate wetting and drying in 0.5% NaCl solutions suggested that the lower

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corrosion was the result of the slower drop in pH at the corroding interface. This was assumed to be the result of the role of Al, Ni and Si alloys in the formation of fine grained and thus less permeable rusts [7].

Because the addition of Al to Fe tends to reduce ductility [8], particularly for additions greater than 10% (mass), mostly Al has been used in combination with Cr to produce FeCrAl alloys that have high heat resistance. Durability testing has been confined largely to high temperature oxidation testing for short-term exposures [9]. However, there is continuing interest in the use of Al to produce light-weight high strength steels, particularly for applications such as in the motor vehicle industry [8] [10]. A number of patented alloys have been proposed and attention has been given mainly to ductility and strength considerations, with a passing note about the potential for greater corrosion resistance, but no actual corrosion data from either laboratory or field trials appears to be available. For steels, microbiologically influenced corrosion (MIC) has been implicated for steels exposed to natural seawater environments, including atmospheric corrosion [11] [12]. Whether MIC is a significant factor also for steels with Al content is of interest, since it is known that Al has a high degree of toxicity to microorganisms [13]. An early laboratory study by Videla et al. [14] of various metals including Al exposed to natural flowing seawater showed evidence of biofilm and bacterial attachment to the metal surface. Subsequent controlled laboratory experiments showed 'sparsely distributed areas of pitting' after 7 days, attributed, after potential measurements, to the effect of microfouling. An even earlier study by Tiller and Booth (1968) using both polarisation resistance measurements and mass loss experiments over 12 months, both in NaCl solutions, concluded that batch cultures of sulphate reducing bacteria increase the corrosion of aluminium manifold. They attributed this to the production of aggressive sulphides and that these have similar effects for both steel and for aluminium. The following describes laboratory and field trials of the marine environment corrosion performance of steel alloyed with aluminium additions of 0% (control), 2%, 4%, 6% and 8%. Although the Al-Fe phase diagram is rather complex [16], within the range of Al addition of 0-10% negligible change occurs in the liquidus/solidus balance, indicating that any effect the addition of Al may have over this range is not influenced by compositional phase effects.

Testing programme - coupons

Test Coupons Five 20 kilogram heats of low carbon steel were cast with increasing amounts of aluminium added with each heat. The riser sprues were removed and half of each block slit into wafers that were used for other testing. Significant voids consisting mainly of shrinkage faults were observed in the slit wafers. To close these voids the retained halves were forged into approximately 12 mm x 50 mm thick blanks. Corrosion test coupons were machined from these forged blanks. There was no evidence of voids in the forged material. Composition of the five materials is set out in Table 1.

Typically, corrosion evaluation coupons are cut from thin sheet (1.0 – 3.5 mm thick) where the width of the perimeter is insignificant and not taken into consideration during mass loss calculations. Also coupons are generally guillotined from a sheet using a backstop resulting in the overall dimensions being almost identical (within 0.1 %). As noted above, the cast steel coupons for the present experiment were forged to approximately 12.0 mm blanks and individually machined to flatness. Coupon dimensions varied from 9.7 to 13.3 mm in thickness, 43.8 to 48.8 mm in width and 73.8 to 78.2 mm in length. Because of these

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variations in size, all coupons in the present test series were individually measured and the separate surface areas derived.

Table 1. Analyses of cast steel coupons

Al → content

0% 2% 4% 6% 8%

Sample series →

0 2 4 6 8

Element ↓ Al 0.013 1.96 3.7 5.8 7.55

C 0.13 0.17 0.17 0.15 0.15

Si 0.44 0.52 0.45 0.44 0.39 Mn 0.54 0.64 0.56 0.53 0.50 S 0.007 0.007 0.010 0.010 0.008 P 0.017 0.021 0.019 0.016 0.018 Cr 0.11 0.12 0.11 0.10 0.10 Ni 0.02 0.03 0.02 0.02 0.02 Mo 0.07 0.08 0.07 0.07 0.07 Cu 0.02 0.02 0.02 0.02 0.01

The test pieces were deployed in ‘as machined’ condition. When recovered, excess corrosion product was physically removed by wire brushing and then the specimens were cleaned using inhibited 16% HCl. Coupons were weighed to the nearest 0.01 g prior to deployment and again after cleaning. From these readings the average thickness of material loss was calculated. Samples were identified according to the aluminium content, ie cast steel (0% added Al) “0” series numbered 0xx, “2” series – 2% Al added series numbered 2xx, etc.

Testing programme – laboratory results

Hardness Testing Vickers Hardness (HV) testing was conducted using a 30 kg load. Results are shown below.

Table 2: Results of Vickers Hardness testing

Result (HV30)

Sample 0 Sample 2 Sample 4 Sample 6 Sample 8

1 172.5 191.9 185.0 181.5 195.0

2 163.7 176.0 187.9 179.0 186.0

3 180.9 181.0 173.1 177.0 185.0

Average 172 183 182 179 189

Minimal variation in hardness was observed with increasing Aluminium content.

Optical Microscopy Representative micrographs were taken of all samples, at original magnifications of 100x and 500x. These and additional micrographs of interesting features are shown below (Figs. 1-6).

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Figure 1: A) Sample 0, 100x B) Sample 0, 500x

Figure 2: A) Sample 2,100x B) Sample 2, 500x

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Figure 3: A) Sample 4, 100x B) Sample 4, 500x C) Sample 4, 1000x

The low carbon steel without Al addition (Sample 0) displayed a microstructure of slightly banded, polygonal ferrite and pearlite (Figs. 1A and 1B). Addition of ~2% Al resulted in coarsening of the ferrite grains, with increased banding and separation of the pearlite grains (Figs. 2A and 2B).

Figure 4: A) Sample 6,100x B) Sample 6, 500x

Figure 4: C) Sample 6, 500x D) Sample 6, 1000x

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The sample containing ~4% Al had been subjected to large modification of the microstructure. Further coarsening of the ferrite was accompanied by separation of the previously pearlitic regions, into what appeared to be a non-lamellar ferrite/pearlite subgrain structure (Figs. 3A, 3B and 3C). Sample 6 bore no resemblance to any conventional ferrite / pearlite structure. Intermetallic phases were present as networks along ferrite grain boundaries, and also as angular needle/lath-like features within ferrite grains. The dark, elongated features could not be classified; however they appeared to contain at least two phases (Figs. 4A, 4B, 4C and 4D). An intermetallic phase likely to be present is Fe3Al.

Sample 8 displayed another, alternate group of morphologies. There were no discernable grain boundaries within the ferritic matrix, which contained dispersions of discrete intermetallic and / or carbide particles. The individual particles in tightly grouped dispersions had a globular morphology, while those which were not grouped were more angular (Figs. 5A, 5B and 5C). An intermetallic phase likely to be present is Fe3Al.

Figure 5: A) Sample 8, 100x B) Sample 8, 500x C) Sample 8, 500x

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Figure 6: Defect in Sample 8 A) 100x B) 500x

The unetched cross-section of sample 8 showed that it contained a seam-like defect feature. The discontinuity was observed to be filled with a non-metallic or intermetallic phase, and displayed numerous branches. The defect feature possibly is the result of manufacturing issues or a lack of homogeneity / solubility [1] in the original melt (Figs. 6A and 6B).

Field testing programme – test sites

Test Sites Sets of coupons were deployed in both atmospheric and immersion conditions. The atmospheric sites were Belmont Beach and North Bondi. Samples were exposed for one year at both of these locations. The samples for immersion testing at Jervis Bay and Taylors Beach were recovered after 1, 2 and 3 years exposure. At North Bondi (Sydney Australia) [S 33º 53’12” E 151º 17’24”] , samples were exposed on a rock shelf about 10 metres from breaking surf. Weather conditions at this location are similar to that at Belmont with typical temperatures ranging from 8.0°C to 25.9°C. Extreme temperatures recorded during the one year exposure reached 46.7°C during January 2011 and dropped to 8.0°C in June 2011. Mean annual rainfall is 1214 mm that occurs during only about 100 days a year. Belmont Beach [S 33º 03’ E 151º 40’ 12”] is a severe marine atmospheric test site located approximately 100 km NE of Sydney, Australia approximately 200m from the breaking surf of the Pacific Ocean. Other results from this test site have been reported previously [17, 18, 19, 20]. At this site the corrosion rate of mild steel consistently has been found to be between 500 µm/y and 800 µm/y. Chloride deposition rate ranges from 250 to 350 mg/m2.d. Time-of-wetness typically is 5650 hr/y (66%) but this will vary from year to year. Air temperature at the site varies from just above freezing in winter to low 40˚C’s in summer, averaging about 22°C annually. For the marine immersion tests, samples were deployed under the Captain’s wharf at HMAS Creswell, the Royal Australian Navy College at Jervis Bay (130 km SW of Sydney, S 35º 07’, E 150º 42’). The establishment is surrounded by Booderee National Park and is located on the shore of Jervis Bay Marine Park. At HMAS Creswell the average water temperature is 18.7°C, ranging from 13.8°C to 24.8°C. The water quality can be considered as pristine pelagic. Analysis for nutrients in the HMAS Creswell water is set out in Table 3, performed by a certified water quality laboratory.

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Samples for immersion tests were exposed also at the Fisheries Research Station at Taylors Beach (S 32º 45’ E 152º 03’). It is approximately 150 km NE of Sydney and is located about 17 km from the Pacific Ocean in the large Port Stephens estuary. The water temperature averages 20.0° C with a low of 11.3°C and a summer high of 29.1°C. The reason for the greater water temperature range at Taylors Beach compared to Jervis Bay is related to the extensive mud flats upstream that heat the shallow water in summer and allow it to cool more in winter. Samples at this site were deployed on the estuarine floor and were slightly covered with silt, so can be considered as below the mud line. Analysis of the water nutrients from this site is set out in Table 3.

Table 3. Nutrient Level at immersion test locations

Nutrient

Ammonia Nitrate Nitrite Total Phosphorous

Site HMAS Creswell

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Atmospheric - Belmont Beach

Results and conditions from the Belmont Beach atmospheric test site have been reported previously [17-20]. Fig. 9 gives a perspective of the proximity to the ocean of the rack on which the cast steel test pieces were exposed. The test pieces were suspended vertically, from a nylon bolt, facing north. Coupons were deployed in November 2010 and recovered 365 days later. Corrosion losses for the 12 month exposure period are shown in Fig. 10.

Figure 9. Rack with cast steel samples at Belmont Beach showing proximity to Pacific Ocean.

Figure 10. First year corrosion loss of different aluminium content cast steels at Belmont Beach.

The effect of alloy content on corrosion loss at the Belmont Beach site is much more pronounced than at Bondi. There is a distinct trend between corrosion loss and aluminium content. Both logarithmic and exponential functions fit the data, in each case with an R2 value of around 0.92. At the Belmont Beach site the reduction on corrosion loss was 58%, 67%, 75% and 81% for the 2, 4, 6 and 8 % Al additions respectively.

Atmospheric - Comparison of the two atmospheric sites Both the Bondi and the Belmont Beach locations can be considered severe marine atmospheric sites. However, because of its proximity to breaking surf, and the substantial wetting of the coupons directly by seawater, the Bondi site could be considered extreme marine. Therefore, it should follow that steel samples exposed 10 m from breaking surf would corrode much faster than those exposed 200 m from the same breaking surf [2]. A comparison of the corrosion loss of the cast steel samples exposed at the two locations is shown in Fig. 11. Simultaneously with the cast steel exposure programme, another exposure comparison using mild steel coupons was undertaken at both atmospheric sites. Coupons were recovered on a monthly basis from Belmont and at irregular intervals from Bondi. The mild steel coupons can be seen in Fig. 7 on the upper right side. Corrosion loss results from the mild steel comparison are shown in Fig. 12.

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Figure 11. Comparison of corrosion loss of aluminium alloyed cast steel at Bondi and Belmont Beach severe marine sites after one year of exposure.

Figure 12. Comparison of corrosion loss of mild steel coupons at Bondi and Belmont Beach severe marine sites.

For the 4%, 6% and 8% alloy compositions there is little difference of corrosion loss between the two locations, however the 2% Al coupons lost about 50% more material at Bondi compared to the Belmont site. Moreover, it is noted that the mild steel exposed at these two sites lost 13% more material at the Belmont site compared to Bondi.

Immersion - HMAS Creswell at Jervis Bay As shown in Table 3, the seawater at the Jervis Bay site is essentially pristine with no significant nutrient levels. The coupons were suspended under the wharf shown in Fig. 13. Coupons were deployed in April 2008 and recovered annually for the next three years. Corrosion losses for the aluminium-alloyed steels over the three years are shown in Fig. 14.

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Figure 13. Captains wharf at HMAS Creswell, Jervis Bay, used for field exposure studies.

Figure 14. Corrosion loss of steel alloyed with aluminium at 1, 2 and 3 years exposure at Jervis Bay.

Corrosion loss after one year showed a definite trend indicating increased corrosion resistance with increase aluminium content. Also, corrosion loss was similar for the unalloyed steel and 2% Al content, both losing about 120 microns. The corrosion losses for the 6% Al and for the 8% Al were similar, about 50 microns. After two years the trend was more pronounced with a decrease in loss for every step increase in alloy content. Overall, there was a fourfold increase in corrosion loss between the unalloyed steel (0%) and the 8% aluminium alloy. The results for the recovery at 3 years followed a similar trend and showed a progressive increase in corrosion loss between the unalloyed steel and the alloyed steels. After three years the 8% alloy had corroded less than a quarter of that of the unalloyed steel. The greatest single step difference was between the 2% and 4% Al content with an increase of 2% alloy resulting in a more than halving of corrosion loss.

Immersion - Taylors Beach In parallel to those deployed at Jervis Bay, aluminium alloyed coupons were deployed in April 2008. They were recovered on an annual basis. Instead of the usual red/orange corrosion product typically observed for mild and low alloy steels, when these coupons were covered they were covered with a black, gritty sludge (Fig. 15). The primary reason was that these coupons were set, initially, on the mud floor of the exposure site and, with time, sank slightly into the mud below, the Taylors Beach wharf (Fig. 16). Corrosion loss for the steel exposed at Taylors Beach is shown in Figure 17.

Figure 15. Coupons from just below the mud line at Taylors Beach, after 2 years exposure.

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Figure 16. Wharf at Taylors Beach from which coupons were deployed.

Figure 17. Corrosion loss of steel alloyed with aluminium exposed just below the mud-line at Taylors Beach.

Comparison of all test environments

To assess the relative severity of the various environments in which the test pieces were exposed all the results are summarized in Fig. 18 for one year exposures and in Fig. 19 for all the results.

Figure 18. Corrosion losses of alloyed steel at all four sites after one year of exposure (JB = Jervis Bay, TB = Taylors Beach).

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Figure 19. Corrosion losses for all coupons for all sites (JB = Jervis bay, TB = Taylors Beach). Both Bondi and Belmont are for 1 year of exposure.

Discussion

Overall the results are generally consistent and in accord with the expectation that a higher degree of alloying with aluminium increases corrosion resistance. But this is not entirely true across all cases. Fig. 11 shows that for the 4%, 6% and 8% alloy compositions there is little difference in corrosion loss between the two severe marine atmospheric exposure locations Bondi and Belmont Beach. The 2% Al coupons lost about 50% more material at Bondi compared to the Belmont site. Moreover, the steel unalloyed with aluminium lost 13% more material at the Belmont site. The reason for these fluctuations may be associated with the microstructure of the alloyed metal, a matter that will be pursued in due course. Despite the differences in atmospheric corrosion severity of the Bondi site (within 10 metres of breaking surf) and the Belmont site (200 metres from breaking surf), it is noteworthy that overall the longer term corrosion losses were not very different. The corrosion losses at both sites are generally the same but were slower to progress initially in the more severe environment (Bondi). As noted, this observation was confirmed by the corrosion loss results for mild steel coupons exposed at both locations. The results shown in Fig. 14 indicate that similar to atmospheric corrosion conditions, the addition of aluminium significantly reduced corrosion in immersion conditions. The results shown in Fig. 17 are for corrosion losses below the mud line at Taylors Beach. It is evident that corrosion loss below the mud line is significantly less than that in the free flowing waters, such as at Jervis Bay (Fig. 14). Also, there is relatively little difference in corrosion losses between the different alloys. This is understandable as the mud would have offered some degree of control over access by oxygen, and also anoxic corrosion conditions may have developed more readily. After one year the 6% alloy showed the greatest resistance to corrosion, corroding only 36 microns compared with the other four alloys that had between 54 and 68 microns corrosion loss. Again, after two and three year’s exposure the 6% alloy had

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corroded less than the other four alloys. Consistent with the other results, in this case too, the overall three-year trend showed that increased inclusion of aluminium reduced corrosion loss. As noted, the reason for the lower corrosion loss under the mud line is probably the result of restricted availability of oxygen, in part the result of the coupons being covered with silt. In part, also, oxygen access may have been restricted by the black crust that was observed to have formed as part of the corrosion product. It appeared to be quite adherent and to consist of a thick silt encrustation.

Figs. 18 and 19 essentially summarize the results of the test programme. In almost all cases the steel unalloyed with aluminium corroded more than the steels with aluminium alloying. Ironically, the most corrosive environment for steel unalloyed with aluminium was that furthest from the water (at only 200 metres). When comparing the steel unalloyed with aluminium and 2 % Al steel after one year, it appears that the more severe the environment the greater the effect of the small additions of alloy, to the point where at Taylors Beach the 2% Al corroded slightly more than the steel nominally without aluminium alloying. Corrosion loss for the 4% alloy was not very different at the four locations and it corroded least at the otherwise least corrosive exposure site. The addition of 6% Al had the least effect at the highest corrosive location where the corrosion loss was 59% of that of the steel unalloyed with aluminium compared to 52% at Taylors Beach, 42% at Bondi and 41% at Jervis Bay. The results indicate that the corrosion loss for the 8% alloy is a function of the corrosivity of the location. At Belmont corrosion loss of the 8% alloy was 19% of that of steel unalloyed with aluminium. At Bondi the high alloy content reduced corrosion of steel to 25% and at Jervis Bay corrosion loss was 41% of the steel unalloyed with aluminium. The high alloy content had almost no effect in the below mud-line conditions where corrosion loss of the 8% Al metal was 98% that of the steel unalloyed with aluminium.

Conclusion The addition of aluminium up to 8% to steel increases corrosion resistance, which for atmospheric and immersion exposure conditions resulted in a reduction in corrosion loss of up to 19% over 3 years exposure at the most severe exposure site.

In immersion conditions the effect of alloy addition appeared to be more significant for longer exposure periods (up to 3 years).

Under mud-line conditions the addition of Al generally reduced corrosion but not to the extent observed in marine atmospheric and immersion environments. There was only a moderate effect on corrosion resistance with increasing aluminium content.

Acknowledgments

The authors acknowledge the Hunter Water Board, The Sydney Water Board, The NSW Department of Primary Industries at Taylors Beach and the Department of Defence at HMAS Creswell for providing facilities for the field trails described herein. Continued funding by the Australian Research Council is gratefully acknowledged.

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