CORROSION OF N I C K E L

RESISTANCE

OF S T E E L S

IN A G G R E S S I V E

WITH

A LOW C O N C E N T R A T I O N

MEDIA

(UDC 669.14.018.84 : 621,785) N. A. L a n g e r , L. N. Y a g u p o l ' s k a y a , K. A. Y u s h c h e n k o , V. G. F a r t u s h n y i ,

N. I. K a k h o v s k i i , a n d G. I. C h a l y u k

E. O. Paton Institute of Electric Welding of the A c a d e m y of Sciences, UkrSSR Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 29-32, February, 1966

In this article we describe the results of our investigation of the influence of the c h e m i c a l composition of stainless steels with a low nickel concentration on their corrosion resistance in solutions containing chlorine ions. The mechanism of pitting corrosion was related in [t] and [2] to disruption of passivity. Therefore, it is of interest to investigate the behavior of low nickel steels in nitric acid solutions. We studied c o m m e r c i a l batches of steels with a low nickel concentration (see Table 1 and Fig. 1). For c o m parison, we also studied 1Khl8N10T steel. Pitting corrosion was studied by immersing the samples c o m p l e t e l y in a 0.5 N solution of iron chloride [3]. The potential at which the protective film is broken through was determined during anodic polarization in a 3% solution of NaC1. The corrosion rate was determined in boiling 20~ nitric acid solution; the steady potential was measured in the same solution over a period of 24 h. The samples were tested in the 0.5 N solution of iron chloride for 37 days. The samples were degreased and immersed in the solution at 18-21~ The tests showed that pitting occurs on Khl4G14N3T (EI711) steel after 6 days, on KhlTAG{4 after 23 days, and on 0Kh21N3T after 34 days. 0Kh21N6M2T steel turned out to be insusceptible to pitting corrosion and retained its smooth surface after 37 days of testing. During the same testing period there was no pitting of 0Kh21N5T steel, but characteristic local subsurface corrosion developed. We assume that this corrosion is due to the local defects formed during rolting of the sheets. Thus, 0Kh21N6M2T steel is the most resistant to pitting corrosion. The breakthrough potential of theprotective film during anodic polarization was determined by the method described in [3]. Before the measurements, the surfaces of the samples were cleaned with sandpaper, degreased in dichloroethane and methyl alcohol, etched 5 rain in aqua regia, washed in water to remove the reaction products, TABLE 1 Concentration of elements, % 8teel No.

0Kh21N3T 0Kh21N5T

(~P214) (EP53~

0Kh21N6M2T *~EP54) Khl4G14N3T~(EI711) KhlTAG14,(EP213)

0,05 0,06 0,06 0,09 0,07

Ti

Mn

Sl

Cr

Ni

0,6 0,7 0,7 13,6 14,1

0,4 0,6 0,4 0,3 0,5

20,8 20,5 20,7 14,4 17,8

3,2

0,3

5,3

0,5

6,0 3,1 0,2

0,4 0,5 --

oT ! : L i

aH, kgm/cm 2

kg/mm2 ! 50 45 49 25 55

64,4 80,2 77,4 80,0 82,7

25,0 27,4 25,6 70, 1 46,6

4,0 14,0 11,3 18,0 15,4

*2.3% Mo.

,o.259

N.

121

Fig, 1. Microstructure of steels as delivered, x 150, a) Kh17AG14; b) Khl4G14N3T; c) 0Kh21N5T; d) 0Kh21N6M2T. V

/

~5

/

a*

!

2 A

/,

The results show that of the steels investigated the Khl4G14N3T steel is the most susceptible to pitting corrosion and the 0Kh21N6M2T steel is the least susceptible. These resuits agree with those obtained in a 0.5 N solution of FeC1a. Comparison of the values of the steady potentials shows that 0Kh21N6M2T steel is more susceptible to passivation in a 0.5 N solution of NaC1 than the KhlTAG14 and Kh14G14N3T steels.

~z

0

o

--

. . . .

:2

/ j. 1 /

j

?

Voltage Fig. 2. Anodic polarization curves of steel samples in 0.5 N NaC1. 1) 0Kh21N3T; 2) 0Kh21N5T;3) 0Kh21N6M2T 4) Kh17AG14; 5) Kh15G14N3T. TABLE 2

Steel No.

0Kh21N6M2T 0Kh21N5T 0Kh21N3T KhlTAG14 Khl4G14NgT IKhI8NIOT

122

Potential in 3% NaC1 Breakthrough Steady @0,016 .--0,077 --0,038 --0,172 --0,146 ~-0,006

and then washed again in distilled water. After this treatment, the samples were placed in glass vessels for polarization and measurements of the potentials. The sample holder was made of Teflon. The potentials were measured with the A4-M2 cathode voltmeter. The tests were made at 25~ The value of the potentials (Table 2 and Fig. 2) are given with respect to the normal hydrogen electrode, taking diffusion potentials into account.

O, 563 0,428 0,298 O, 193 O, 023 0,360

The analysis of the results leads us to the following conclusions relative to the singularity of the behavior of low nickel steels. The concentrations of 21% Cr, 6% Ni, and 2% Mo e n sure high resistance to pitting corrosion, as indicated by the high positive breakthrough potential, Ep, equal to 0.563 V. When there is no molybdenum and the nickel concentration is 0.7% lower the breakthrough potential of 0Kh21NST steel is Ep = 0.428 V. The decrease in the concentration of nickel to 3.2% decreases the value of Ep even more. The decrease in the concentration of chromium to 17% and the replacement of nickel with m a n g a n ese or nitrogen (KhlTAg14 steel) decreases E to 0.193 V. The even in the decrease of the chromium concentration to presence of 3.1% Ni, decreases Ep to 0.023 V in spite of the homogeneous austenitic structure (Fig. la, b). The breakthrough potential and, consequently, the resistance to pitting corrosion of Kh14G14N3T steel is lower than that of Khl8N10T steel.

lP4%,

The relationships described here indicate that a decrease of the chromium concentration decreases the resistance of the

TABLE 3 Corrosion rate in boiling 20% HNOa, h

Steel No.

g/m2.

0Kh21NOM2T 0Kh21NST 0Kh21N3T Khl7AG14 Khl4G14N3T

1,180 1,070 1,020 0,880 0,820

0,022 0,046 0,055 0,143 0,157 0,020

I K h I 8 N 10T

Potential, V, after 24 h

steel to pitting corrosion much more than a decrease in the nickel concentration and more than the existence of a twophase structure in the steel. 0Kh21N5T and 0KhglN6M2T s t e e r have a high resistance to pitting corrosion and in this respect are as good as c h r o m i u m - n i c k e l austenitic steels of the 18-8 type (0Kh21N6M2T steel is even better). Also, the presence of a two-phase structure (Fig. lc, d) in the materials investigated has much less influence on the corrosion resistance than alloying of the steels with chromium, molybdenum, and nickel. With increasing concentrations of these elements the resistance of the steel to pitting corrosion increases - a result in agreement with that obtained in [4-7].

V

Corrosion tests of 0Kh21NST, 0Kh21N6M2T, and Khl4G14N3T steels in sea water for 4 months gave positive results. * Under conditions of rapid flow (600-800 m / r a i n ) these steels and their welded joints have satisfactory resistance to pitting corrosion. Even Kh14GI4N3T steel did not show any pitting corrosion under these conditions. It should be kept in mind, however, that 4 months is an insufficient period to put in evidence pitting corrosion, ,and therefore these tests are being continued.

t i80

1.too

"~, 9

'6

1.020 o.9,o 0,860

The results obtained show that the steels investigated are passive in solutions containing chlorine ions. No less important Time are the electrochemical characteristics of these alloys in nitric Fig. 3. Dependence of the steady potentials acid. We used a boiling 20%0 nitric acid solution.? This soluof steels on the time in boiting 20% nitric tion has passivating properties but is not a strong oxidizing acid (the notations are the same as inFig. 2). agent [4]. Table 3 gives the data on the corrosion rates of the steels subjected to a 20% HNOa solution and the values of their potentials, which become stabilized after 24 h. Figure 3 shows the dependence of the potentials on time. O,faO -

-

~

3

f2

~6

20

The data in Table 3 show that the 0Kh21N3T, 0Kh21NST, and 0Kh21N6M2T steels have a high corrosion resistance (equivalent to that of 1Khl8N10T steel) in the medium investigated and belong to the group of resistant metals as specified in GOST 5272-50. However, in boiling nitric acid of higher concentrations the corrosion resistance of the steel containing molybdenum (0Kh21N6M2T) is considerably lower than that of the steel without molybdenum. KhlTAGI4 and Kh14G14N3T steels are iess resistant in the medium investigated, as is indicated by more negative values of the potentials and high corrosion rates. The low corrosion resistance of these steels is due essentially to the lower chromium concentration. CONCLUSIONS 1. We have shown that the resistance of homogeneous f e r r i t i c - a u s t e n i t i c stainless steels to pitting corrosion is ensured by increasing the chromium concentration and by additional alloying with molybdenum. 2. We have shown the possibility of replacing c h r o m i u m - n i c k e i steels of the Khl8NI0T type with steels with a lower nickel concentration in several corrosive media (sea water, iron chloride, dilute nitric acid, etc.). LITERATURE 1. 2.

CITED

U. Evans, "I. Electrochem. Soc.," (1956), No. 1. H. Grafen, "Metalloberflache," (1959), Vol. 3, No. 6.

*These studies were made in cooperation with TsKB, Krasnoe Sormovo Plant. J-More extensive studies of the effects of the concentration and temperature of nitric acid on the corrosion resistance of the steels were m a d e in [8, 9].

123

3. 4. 5. 6. 7. 8. 9.

124

N . D . Tomashov, et al., "Influence of alloyed elements on the susceptibility of stainless c h r o m i u m - n i c k e l steels to pitting corrosion," Co11.: Corrosion of Metals and Alloys [in Russian], Moscow, Metallurgizdat (1963). G . V . Akimov, Fundamentals of the Study of Corrosion and Protection of Metals [in Russianj, Moscow, Metallurgizdat (1946). W.S. Flint and W. I. Tott, "Metalhrgia," (1955), Vol. 51. N . P . Talov, "High strength widely available steels," in Coll.: Transactions of the Central Scientific Research Institute of Ferrous Metallurgy [in Russian], Moscow, Metallurgizdat (1960). M . M . Kurtepov and E. F. Bochkareva, "Corrosion of stainless steels in solutions of iron chloride," in Coll.: Corrosion of Metals and Alloys [in Russian], Moscow, Metallurgizdat (1963). N . I . Kakhovskii, K. A. Yushchenko, Z. V. Yushkevich, and Z. F. Istrina, Avtomaticheskaya svarka (1962), No. 11. N . I . Kakhovskii, et al., Avtomaticheskaya svarka (1963), No. 12.