Materials Science, Vol. 30, No. 1, 1994

FORMATION OF DEFECTS IN MATERIALS ATTACKED BY AGGRESSIVE MEDIA B. G. Strongin and A. B. Oleinich

UDC 620.193:539.67

In studying corrosive processes in materials and alloys, researchers traditionally restrict themselves to chemical and physicochemical aspects of the problem [1]. On the other hand, it is known that corrosion induces damages not only on the surface but also in the matrix of a material, i.e., affects its strength properties. This problem is studied insufficiently [2] but proves to be quite important not just from the viewpoint of applications. Thus, it is necessary to study structural changes in materials subjected to corrosion and, above all, the interaction between various defects of the crystal structure and also the first stages of fracture. To solve the problem under consideration, we used the method of low-frequency internal friction. Internal friction and the squared frequency of free torsional oscillations (f2), which is proportional to the effective shear modulus Gef, were recorded by a method suggested in [3] and modified by the authors who used a special accessory for investigating materials in contact with aggressive media. For this purpose, we used a steel adapter fastened in the lower clamp and a reservoir for the liquid medium screwed on the adapter; the reservoir was made in the form of a tube long enough to guarantee complete immersion of the tested specimens in the medium. This enabled us to study not only the influence of corrosive media on the elastic properties of materials but also the influence of external mechanical factors on the corrosion process (cycling with various frequencies and amplitudes of deformation, static tensile loads, etc.). We investigated iron specimens in a simulator of seawater (a 3 % NaC1 solution). The specimens were pieces of wire with a diameter of 2 mm and a length of 50 mm made of Armco iron annealed at 1000°C for 2 h. Internal friction and f 2 were recorded at room temperature in an external magnetic field with H _ 22300 A/m with a corrosion time base of about 240 h, except for the case of free corrosion (about 1000 h). The accuracy of the measurements of f 2 and the logarithmic decrement of damped oscillations Q- 1 was not worse than 0. 1% and 1%, respectively. All the specimens were divided into three groups. The first group was subjected to "free" corrosion, the second to corrosion under cyclic loading by torsion in a relaxometer in measuring the amplitude dependence of internal friction (i.e., under cycling with a strain amplitude increasing from 2- 10-5 to 2 • 10 -4, and the third group was subjected to corrosion under additional cycling (as compared to the second group) with a constant strain amplitude (_ 10-4) for several dozen minutes every 12 h. Before starting the tests, all the specimens were weighed (with an accuracy of + 0.1 rag) and parallel monitoring of corrosion by the gravimetric method was realized. In addition, in measuring f2, the mass losses were recorded from the changes in f as a function of the corrosion time t on the basis of the expression (mt - m o ) / m o = ( f t - f o ) / f o , where m t, m 0, ~, and f0 are the masses and the frequencies of free oscillations of the specimen at the times t and t o = 0, respectively. Results of the Tests and Discussion The shape of the dependences Q - l ( y ) and f2(,/) changes significantly with the time of corrosion but the character of these changes is different in different regions (Fig. 1). For a base of 0 - 75 h (the first stage), friction increases insignificantly and the curves of the amplitude dependence of internal friction practically coincide. On the 1 These relations were derived from the dependence f 2 = Gd 4 / (1.28 r~ll), where f is the frequency, G is the shear modulus, d and l are the diameter and the length of the specimen, and I is the moment of inertia of the torsion system, under the assumption of weak variation of I with t. As shown by control weighing, the values of A m / m found by ~is method coincide with gravimetric results with an accuracy of at least 0.5 - 1%. Chernivtsi State University, Chernivtsi. Translated from Fiziko-KhimicheskayaMekhanikaMaterialov,Vol. 30, No. 1, pp. 132-I34, January-February, 1994. Original article submitted October 8, 1992. 1068-820X/94/3001-0141 $12.50

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B . G . STRONGIN AND A. B. OLEINICH

basis of the dislocation model of the amplitude dependence of internal friction [4], this type of damping behavior can be explained by small changes in the dislocation-dopant structure of the matrix of the specimen. On the other hand, the slight decrease in the modulus Gef indicates that the dislocation mobility increases with the amplitude of the stresses; this can be explained by weakening of the dislocation-stopper bond. For a base of 75 - 240 h (the second stage), the dependences have the opposite behavior, namely, absorption decreases while Gef increases, indicating that the material is strengthened (probably due to reanchoring of dislocations). This result agrees with data in [5], where it was shown that steels subjected to corrosion in a 3 % NaC1 solution under a low-frequency load (10-2 _ 102 Hz) undergo intergranular hydrogen-induced cracking, i.e., hydrogen embrittlement. An analysis of our data demonstrates that embrittlement begins not immediately but only in the second stage of corrosion and is caused not by nucleation of new linear defects (the internal friction background is preserved) but by arrest of existing dislocations by aggregates of hydrogen emerging within the first period of corrosion. l~"t

!

I

,

5.10-~

t. lO~

@

• I

Fig. 1. Amplitude dependences of Q-: (a) and f2 (b) for specimens from the second group for various exposures in a 3% NaC1 solution: 1 - 0 h ; 2 - 5 2 h ; 3 - 7 5 h ; 4 - 144h; 5-194h; 6-216h; 7-240h.

The two-stage nature of corrosion for specimens from this group is also corroborated by data on the mass losses Am/m (Fig. 2, curve 2). As can be seen in Fig. 2, the curve zXmlm(t) can be split into two parts with different corrosion rates vI2 = 0.8 g/(m 2. h) in the interval 0 - 75 h and v~ = 4.853 g/(m 2. h) in the interval 7 5 - 240 h; this can be regarded as an additional argument for the existence of two mechanisms of corrosion [1]. Furthermore, the corrosion of specimens from the second group differs from the free corrosion of specimens from the first group (Fig. 2, curve 1). In the first case, active corrosion began immediately after the specimens had been placed in a solution while, in the second case, we observed a lag period of about 100 h. Active corrosion was initiated only after this period and its rate v I = 0.038 g/(m 2. h) remained constant for about 1000h and differs significantly from v I and viI. This property manifests itself more sharply for specimens from the third group. The additional cycling increases the corrosion rate in the second stage by a factor of 1.4. In this case, v~ was practically constant, and the corrosion mechanism changed several dozen hours earlier than for specimens from the second group. The increase in the corrosion rate under the additional cycling confirms the conjecture on the hydrogen mechanism of embrittlement in the second stage. This agrees with data presented in [7] indicating that, under small constant o r cyclic deformations, accumulation of hydrogen at crack tips accelerates and affects the kinetics of hydrogen embrittlement substantially.

FORMATION OF DEFECTS IN MATERIALS ATTACKED BY AGGRESSIVE MEDIA

143

For specimens from the third group, the amplitude dependences of internal friction and the amplitude dependence of f 2

have almost the same form as those depicted in Fig. 1, namely, a weak "softening" (decrease) in the

modulus Gel is followed by strengthening. However, the change in the mechanisms occurs earlier than for specimens f r o m the second group. It seems that even small cyclic loads (_ 1 0 - 1 2 M P a ) accelerate corrosion processes considerably.

0.1 6.0

3'/ 0,05

5.0

0

tO0

goo

t, h

Fig. 2. Dependences of relative mass losses on corrosion time for specimens from the first-third groups (1 - 3, respectively); ~ ' - 3 ' - the corresponding values of f(t). Numerous investigations of the influence of dynamic and cyclic loads on corrosion processes [ 5 - 7] confirm what has been said above since they also indicate considerable acceleration of corrosion of iron and its alloys precisely under a low-frequency load. Note that the mechanisms of this acceleration have not been clarified completely yet. Therefore, it seems helpful to collect data on the behavior of structural defects in materials in the course of these processes.

REFERENCES 1. H. Kaesche, Metal Corrosion. Physico-Chemical Principles and Actual Problems [Russian translation], Metallurgiya, Moscow (1982). 2. S. Mrovec and T. Verbeure, Contemporary Heat-Resistant Materials [Russian translation], Metallurgiya, Moscow (1986). 3. B.G. Strongin, I. A. Varvus, and P. A. Yakovyshyn, An Improved Device for Measuring Elastic Energy Damping Characteristics, Deposited at UkrNIINTI, 08.07.93, No. 798 Ukd 83, Chernivtsi University, Chemivtsi (1983). 4. M.A. Krishtal and S. A. Golovin, Internal Friction and the Structure of Metals [in Russian], Metallurgiya, Moscow (1976). 5. Shimodaira Masuo et al., "Acceleration of fatigue crack growth in NT80 and SUS 304 steels in a 3% NaC1 solution under extremely low load frequencies," J. Soc. Mater. Sci. Jpn., 39, No. 37, 162-168 (1990). 6. K. Komai et al., "Dynamic and cyclic stress corrosion cracking resistance of metals," in: Adv. Materials Severe Service Appl. Proceedings of the Japan-USA Scientific Seminar (Tokyo, 1986), London, New York (1987), pp. 373-383. 7. S.R. Scully and P. J. Moran, "The influence of strain on hydrogen entry and transport in a high strength steel in sodium chloride solution," J. Electrochem. Soc., 135, No. 6, 1337-1348 (1988).