CORROSION USED

RESISTANCE

IN HYDRAULIC

OF ALUMINUM

ALLOYS

CONSTRUCTION

G. M. Akhmedov, V. I. Shakov, M. S. Trifel', and A. G. Khanlarova

UDC

627.8:620.193

Considerable expenditure is required to combat corrosion of steel hydraulic structures and equipment in rivers. Until recently, the chief method of suppressing such corrosion was the use of resistant enamels or paints. Owing to the considerable weights of equipment in hydroelectric power stations and the difficulties of dismantling, transportation and reassembly, any solution which will eliminate anticorrosive treatment of large items ofplantsmust be attractive. Extensive research has therefore been made into the possibility of electrochemically protecting submerged metal structures so as to dispense with time-consuming painting work. This method should be very popular for protecting steel components in hydraulic engineering structures. Construction engineers will be interested in techniques involving corrosion-resistant structural materials, especially aluminum alloys, with the aim of prolonging service life. Their high specific strengths and resistances enable such alloys to protect structures from corrosion and greatly reduce their weights; assembly is easier and hoisting facilities need not be so extensive. The present paper gives the results of a program of research at the Gipromorneft' Institute (Baku), aimed at correlating the mechanical and corrosion properties of aluminum alloys under conditions imposed by the average flow of the Volga at the V. I. Lenin Hydroelectric Power Station and in the Caspian Sea. The program embraced the severest corrosion conditions so as to provide answers of practical importance for a number of other regions in the USSIL Table 1 gives the chemical composition of the water, together with the climatic factors for the trials. Tested were Amg3M, AMgSV, and D16AT commercial alloys, all widely used as structural materials, and also A1M aluminum as a metal covering; their chemical compositions and properties are given in Table 2. TABLE 1. Chemical Composition of Fresh Water at the V. I. Lenin Hydroelectric Power Station on the Volga and the Water of the Caspian Sea, and Climatic Factors of the Test Regions Ions. m ~/liter

Climatic factors temperature, de~. relative moisture

_

Test conditions

rainfall, ram/yr

max. |min.

max. rain.

+16,7 --13,6

20,0

max. min.

G Marine At the Vol.ga Hydroelec me

329 45

702 13

3 271 o0

2 933 5 481 68 29

220 [ 1 123

19,3 43,2

84

station

TABLE 2. Chemical Composition and Mechanical Properties of Aluminum Alloys Chemical c o m ~ _ ( r e m a i n d e r Alloy mark AIM AMg3M AMgSV D16AT

Cu

0.015 <0.05 ~0,05 3.8--4.9

Mg

Si

0,3

I 0,3

4,8-5,5 / 4o,5

t
1,2--1,8 I ~ 0 , 5

[ --

3,2----3,8

I

Fe

0,5--0,8 [<0,4

AI), _ ~ _ ~ Mn

Zn

other

0,3~0,6 -<~,21 < O , l 40,2 40,1 0,3--0,6 0.3--0.9 <~0,3 4 0 , 1

Mechanical properties (not less than

~0,2, at . kg/mm z kg/mm ~ g

2 28 43

Translated from Gidrotekhnicheskoe Stroitel'stvo. No. 3, pp. 21-24. March. 1968.

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57

G. M. AKHMEDOV ET AL.

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Fig. 1. Change in the corrosion rates of a l u m i num alloys under conditions at the Volga Hydroelectric Station, Test zones: A) atmosphere; 13) variable level; C) submerged. Aluminum alloys: 1)A1M; 2)AMg3M; 3)D16AT.

Fig. g. Change in the corrosion rates of alumin u m alloys under Caspian Sea conditions;

f3) zone of intermittent wetting; 4) alloy AMgSV; for meaning of other symbols, see Fig. 1.

TABLE 3. Mechanical Strengths of Aluminum Alloys after Field Tests at the V. L Lenin Hydroelectric Station on the Volga and at the Marine Corrosion Station of the Gipromorneft' Institute Alloy mark AIM AMg3M AMgSV D16AT

Strength before tes kg/mm 2 8.5 22,3

30,8 32.0

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Figures 1-3 show the corrosion rates for aluminum alloys and carbon steel for submerged zones, in zones of intermittent wetting, and ha the atmosphere, as established for both test regions; in every case, in fresh water the rate of corrosion of the alloys is of the order of a tenth of that in seawater and a tenth or a hundredth of that of s t e e l Aluminum corrodes evenly without deep localized pitting. The propagation of minor pitting which occurred on the aluminum in the intermittent wetting zone slowed down very rapidly. No significant difference was observed between t h e corrosion of plated D16AT alloy and that of AIM aluminum. Attenuation of the corrosion rate was most striking in the submerged zone. Here, after one year the rate was still l e s s than g /~/year in fresh water and 10 y / y e a r in sea water. In the intermittent zone, the average corrosion rate was always less than 0.06 g / y e a r in fresh water and 0.5 /~/year in sea water. In the atmosphere, corrosion rates were less than 0.08 /l/year above water level for fresh water and 0.7 /a/year above that for sea water. The corrosion rates and the changes in them were practically the same for the different alloys in the freshwater intermittent zone; in the atmosphere, the absolute corrosion rates above the fresh-water level were all small; however, mutual comparison revealed differences by factors of 2-a. In sea water (with complete submersion), in the intermittent zone, and in the marine atmosphere, the ratio between the corrosion rates at first reached 1.5-2. However, after six months the rates had balanced out.

CORROSION RESISTANCE OF ALUMINUM ALLOYS

217

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3. Change in the corrosion rate of steel under Caspian Sea conditions. zones: 1) atmosphere; 2) i n t e r m i t wetting; 3) submerged.

Fig. 4. Polarization Polarization curves: alloy; 3) St 3 steel. 4) AMg3M alloy; 5)

in synthetic fresh water. 1) AMg3M alloy; 2) A1M Ditto, overall curves: A1M alloy; 6) St3steel.

Comparison of the corrosion rates for steel and aluminum alloys in sea water with total submersion, in the intermittent zone, and in the marine atmosphere (Figs. 2, 8) reveals the markedly lower corrosion resistance of steel, although attenuation of corrosion is the same. The corrosion rares for steel l e v e l e d out as follows ( w / y e a r ) : 20 in the atmosphere, 50 in the submerged zone, and 250 in the intermittent zone. Although the corrosion rate of aluminum in the atmosphere and the intermittent zone m a y be of the same order in fresh water and sea water, in the case of steel it is one order higher in the intermittent zone than in the atmosphere. In fresh water, corrosion of submerged steel reaches 400 /1/yr in summer, but falls to a few microns per year in winter.

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Thus, the corrosion resistance of a l u m i n u m in both sea water and fresh water is one to two orders higher than that for steel; this 9 ~ corresponds to their polarization patterns in fresh water and sea water Currentdensity, m A / c m z (Figs. 4, 5): the anodic and cathodic polarization rates for a l u m i Fig. 5. Polarization in water of the Caspian num alloys are considerably higher in current density sectors up to Sea. For meaning of curve numbers, see I m A / d m z than for s t e e l - t h i s explains the high corrosion resistance Fig. 4. afforded by a l u m i n u m alloys. The polarization rates and corrosion resistances of all the alloys tested were the same. Close correlations were obtained between the corrosion rates and overall polarizabilkies of the alloys in sea water and fresh water. In fresh water, high resistance to corrosion coincides with m u c h greater polarizability. 0

0

The polarization curves and corrosion rates illustrate alloy behavior in conditions of sluggish flow. A change in the flow rate can greatly alter the ratio between the polarizabilities of the alloys and thus affect their resistance to corrosion. Therefore, these data are not applicable to hydraulic structures exposed to fluctuating rates of flow (turbine runners, penstocks, turbine chamber facings, etc.). Study of the corrosion rate reveals that, as regards weight loss, aluminum alloys will last out any service per•od for hydraulic engineering structures, without additional anticorrosive treat,menL Owing to the fact that the mechanical strengths of aluminum alloys decrease out of proportion to their corrosive wear, the corrosion tests on a weightloss basis were supplemented by tests to determine changes in the mechanical properties of specimens (Table 3).

218

G.M. AKHMEDOV ET AL.

Comparison of the changes in mechanical strengths of the different alloys with time reveals that the strength of AIM aluminum increases (the rise reaches 15% and differs for the different specimens, the spread being :~5% of the original strength values) in all the test zones for brief exposures (less than 30 days) and then remains virtually unchanged. The scatter is even more marked for AMgM alloy specimens: the rise and fall in strength are in the range 5-10% for fresh water and 3-5% for sea water. No regular change in the alloys' properties, relatable to particular test zones, was detected; increase and reduction of strength were noted over various periods in different zones, and were apparently largely due to the specific properties of the specimens rather than the test conditions. The properties of D16AT plated alloy change in virtually the same way as those of AMg3M alloy; over the test period of one year a slight reduction in strength (not more than 3% of the original value) was noted practically throughout. The relative increase in the strengths of aluminum alloys in fresh water was rather higher than in sea water, and in some instances reached 20% of the original strength. Where the latter was considerable, the absolute increase in strength was several times greater than for AIM alloy. Thus, plating greatly enhances the mechanical properties of the alloys; plated alloys should therefore be widely employed for hydraulic engineering structures. However, the relatively brief period of the tests leaves open the possibility that aluminum alloys may lose strength with time. Our data will be of particular interest with regard to the use of aluminum for covering metal structures; the high corrosion resistande of aluminum, coupled with its decorative aspect, will maintain a structure without additional treatment throughout its service life. Our results indicate that aluminum alloys afford appreciable resistance to corrosion in all areas where structures operate in water; thus, no anticorrosive treatment is required throughout the service life of the structure.