the role of metal cations in improving the inhibitive performance of hexamine on the corrosion of...

Pergamon Corrosion Science, Vol. 37, No. 6, pp. 1005-1019, 1995

Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

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THE ROLE OF METAL CATIONS IN IMPROVING THE INHIBITIVE PERFORMANCE OF HEXAMINE ON THE

CORROSION OF STEEL IN HYDROCHLORIC ACID SOLUTION

D. D. N. SINGH, T. B. SINGH and B. GAUR

Corrosion Protection Division, National Metallurgical Laboratory, Jamshedpur - 831007, India

Abstract-Hexamethylenetctramine (HA) or hexamine (or urotropin) have a moderate inhibitive effect on the corrosion of mild steel in concentrated acid solution (3N) but have a negligible effect in very dilute solutions (N/200) of the acid. Incorporation of Cu*, As3+, Sd and Sn*+ with HA improves its performance, which is synergistic in nature. These additives (except As), however, exhibit an antagonistic effect when tested in dilute acid solutions. Cu* and As3 have the most pronounced effect in 3N acid solution. In N/200 HCI solution, the antagonistic effect is a maximum in the case of St? followed by Sn*+ and CU*+ cations. Weight-loss, electrochemical polarization and zeta potential measurements are performed to understand the mechanism of action of these inhibitors. The positive role played by the cations on the inhibitive performance of HA is due to the formation of anionic complexes with the chloride ions of the acid solution. These anions replace the adsorbed chloride ion from the metal-electrolyte interface owing to their higher affinity toward the interface and help the protonized molecule of HA to bc adsorbed more strongly at the interface. Accumulation of FeCI, in concentrated acid solution lowers the performance of HA to a greater extent (about 100 times) than HA blended with Cu*+. The latter composition also has a substantially stronger inhibitive role on hydrogen absorption by the steel than the former one.

INTRODUCTION

It is now well established that the use of hydrochloric acid in pickling of metals, acidization of oil wells and in cleaning of scales is more economical, efficient and trouble-free, compared to other mineral acids. ~4 The great advantage of this acid over the other acids in cleaning and pickling operations lies in its ability to form metal chlorides which are extremely soluble in aqueous phase, compared to sulfate, nitrate, phosphate, etc. This higher rate of solubility of chloride salts causes the least polarizing effect and does not hamper the rate of reaction. Some salts, e.g. FeCl,, produced as a result of the reaction of scales and acid, have a depolarizing effect on the reaction rate and make the acid solution extremely aggressive towards the base metal. To control this depolarizing effect of metal salts and also the attack of the acid on the base metal surface, the use of efficient inhibitors in hydrochloric acid solutions during the above operations is essential. In acid solutions, the iron surface acquires a positive charge (the zero charge potential of iron in aqueous acid solutions varies from -0.4 V to -0.7 V).5.6 This indicates that an inhibitor capable of providing anionic species in the solution should be a good inhibitor for the system. The chloride ion of hydrochloric acid is adsorbed strongly on the metal surface and makes a

Manuscript received 18 October 1994.

1005

1006 D. D. N. Singh, T. B. Singh and B. Gaur

negatively-charged double layer. Cationic types of species in the solution have a tendency to interact with this double layer and provide corrosion inhibition as a result of the formation of a compact film on the surface. A literature survey reveals that inhibitors having an amino group perform very well for the HCI-iron system, due to the formation of cationic species in the acid solutions:.

R-NH2 + H+ + R-NH,:.

Amongst the different types of amines, hexamine is the most widely studied compound for the dissolution of iron and iron-based alloys in acid solutions.- Its molecule acquires a positive charge in acid solutions and has a moderate inhibitive effect on the corrosion of steel in hydrochloric acid. The theory of synergism on corrosion inhibition indicates that the performance of hexamine can be improved if surfactants having an anionic effect at the interface are introduced with the former one in the acid solutions. Organic anionic surfactants are quite effective in improving the performance of inhibitors. but owing to their higher costs, a higher degree of concentrations is required (to achieve satisfactory inhibition), and their other effects, which complicate further processing of either metal surface or the electrolyte, have necessitated a search for inorganic cations which can produce anionic species in the acid solutions and could achieve synergism with the amines. These ions are less expensive and produce appreciable effects on corrosion inhibition even if used at very low concentrations. This paper describes the use of copper, arsenic, antimony and tin, in their chloride forms, as sources of anionic species, in combination with hexamine to control the dissolution of steels in hydrochloric acid under the influence of different parameters. The same compositions have also been studied for their inhibitive performance in a very dilute solution of HCI (N/200) to explore the possibilities of their use in acidic water. which can be used in recirculating industrial cooling systems. The low pH water (pH = 3.54) can be used in place of plain water. which can minimize scale formation in pipelines, and its corrosive action can be controlled by the incorporation of highly effective inhibitors.

EXPERIMENTAL METHOD

Hot rolled mild steel strips were taken from a single lot and small coupons of six 7.5 x 2.5 x 0.2 cm wcrc cut from it. These wcrc then wet ground on polishing wheel to remove the mill scale. Final polishing was carried out at 60 grit silicon carbide abrasive paper. The samples were degreascd with acctone before exposure to the test clcctrolytcs. A water bath, having a tcmperaturc sensitivity of 2 1C. was used for weight-loss studies. Electrochemical studies wcrc performed using coupons, of circular arca 1 .O cm. of the same steel. A saturated calomel electrode (SCE) and a couple of graphite rods were used as reference and auxiliary electrodes, rcspcctivcly. Polarization experiments were performed by using a potentiodyne analyzer supplied by M/S Petrolyte Instruments Co.. U.S.A.

Hydrogen absorbed by the steel during its dissolution in acid solution was dctermincd on samples ol size I .O x 0.5 x 0.2 cm of the same material as used for the weight-loss and elcctrochcmical tests. The technical details of the experimental method arc described in earlier publications.3.

Zeta potential measurcmcnts wcrc performed on iron powder (particle size 200 mesh) produced electrochemically. Lazer Zcc Meter Model SO1 supplied by M/S Pen Kcm, Inc., NY. U.S.A. was used for this purpose.

AR grade HCI acid was used for the preparation of the test electrolytes. Dilution was made with double distilled water. Hexaminc and metal salts. taken in their chloride forms, were also of the AR grade.

Improving the inhibitive performance of hexamine on corrosion 1007

EXPERIMENTAL RESULTS

Inhibition studies in concentrated acid solutions Determining the optimum concentration of inhibitors. Figure 1 shows the per-

formance of HA on the corrosion of steel in 3N HCl at its different concentrations and temperatures. Here, it is noted that the inhibitor performs quite well at 22C and the corrosion rate comes down to ~750 mdd when used at and above concentrations of 300 ppm. At the elevated temperatures, however, the inhibitive performance of HA deteriorates and attains values of 2500 and 5000 mdd at 40 and 55C, respect- ively. At all the three temperatures studied, the optimum performance of the inhibitor is achieved at a concentration of 300 ppm.

To enhance the performance of inhibitor at its optimum concentration, some metal cations, i.e. Cu2+, As3+, Sb+ and Sn2+, were added with the inhibitor during the corrosion of steel in acid solution. The results are shown in Figs 2-5. It is evident from these figures that the copper and arsenic cations achieve a synergistic effect with hexamine in inhibiting the corrosion rate. This is more pronounced at elevated temperatures (40 and 55C) than at lower temperatures. These cations perform best at a concentration of 10 ppm. The effect of antimony and tin cations is nominal compared to copper and arsenic cations and improvement in the inhibition efficiency of hexamine takes place between 2.5 and 5.0 ppm of these cations.

In order to test whether the optimum concentration determined for the performance of hexamine changes in the presence of metal cations or not, experiments were performed taking different concentrations of hexamine and a fixed concentration (10 ppm) of cations at 55C only. The results are shown in Fig. 6. The optimum performance of hexamine is still observed at a concentration of 300 ppm. In the presence of copper and arsenic ions, the trend is slightly accelerated, with an increase in hexamine concentration above 300 ppm. As3+ retards corrosion rate more effectively (about 2.5 times) than Cu2+, at all the studied concentrations of hexamine.

The effect of the passage of time on the performance of optimized (300 ppm HA

i- 0 100 200 300 400

HA concentration (ppm) 0

Fig. 1. The variation in corrosion rate of steel in 3N HCI with concentration of HA


300ppmHA

*0 I I I

2.5 5.0 1.5 10.0 I Concentration of Cu*+ (ppm)

Fig. 2. The effect of Cu*+ on the corrosion rate of steel in 3N HCI in the presence of 300

ppm of HA

300ppmHA

I .-.3oc._

1

2.5 5.0 I.5 10.0 Concentration of As3+ (ppm)

5

Fig. 3. The effect of As+ on the corrosion rate of steel in 3N HCI in the presence of 300 ppm of HA.


300ppmHA

I l - .-. ._.E._

OO I I I I

2.5 5.0 7.5 10.0 12.5 Concentration of Sb3+ (ppm)

Fig. 4. The effect of Sb3+ on the corrosion rate of steel in 3N HCI in the presence of 300 ppm of HA.

300ppmHA

Concentration of Sn*+ (ppm)

Fig. 5. The effect of Sn2+ on the corrosion rate of steel in 3N HCI in the presence of 300 ppm of HA.

lOI D. D. N. Singh, T. B. Singh and B. tiaur

oi 0 loo 200 300 400 500

HA concentration (ppm)

Fig. 6. The effect of metal cations on the corrosion rate of steel in 3N HCI in the prescncc of different concentrations of HA.

23 C

I - 300ppmHA 2 300ppmHA + I Oppm Cu2+ 3 - 300ppmHA + 1 Oppm As3+ 4 - 300ppmHA + 1Oppm Sb3+ 5 - 300ppmHA + IOppm Sn2+

I I I I J 0.25 I 2 3

Time (h)

Fig. 7. The cffcct of time on the corrosion rate of steel in 3N HCI in the prcscnce of

optimized concentration of the inhibitor.

+ 10 ppm cations) concentration of inhibitors is shown in Fig. 7. In all the cases, it is observed that the inhibitive performance of the compositions increases for times up to 2 h and then becomes almost constant. A considerable degree of change in the performance of the inhibitors is noted at longer durations of the exposure for


I-

I-

, -

1 '0

I I I I I

0.1 0.2 0.3 0.4 0.5 0.6 0.7 FeCI, (M)

Fig. 8. The effect of ferric chloride on the corrosion rate and hydrogen absorption of steel in 3N HCI in the presence of 300 ppm HA.

inhibitor having cations and in their absence. At and above the exposure period of 1 h, for example, the inhibitor having cations inhibits the reaction rate about twice as much as in their absence. This same factor is about 1.3 times when tested at 15 and 30 min.

The effect of ferric chloride on the performance of inhibitors. Accumulation of iron salt (FeCls) in acid solution takes place due to the dissolution of scale. This salt depolarizes the corrosion reaction to a considerable extent. Experiments were, therefore, performed to study the effect of FeCls on the corrosion rate and hydrogen absorption by the steel during its corrosion in the presence of the inhibitors (300 ppm HA and 300 ppm HA + 10 ppm Cu+). Only one cation, i.e. Cu*+ , was taken up for the study with 300 ppm of HA and the results are compared with the performance of HA. The observed data is plotted in Figs 8 and 9. A linear relationship has been noted between the molar concentration of FeC13 and the corrosion rate in plain hexamine, as well as in hexamine having Cu 2+ The rate of increase in corrosion rate . (CR) with molar concentration of FeCl,, i.e. dcR/dlFecI~l, is markedly lower in the case of the inhibitor fortified with Cu2+ than in the plain inhibitor (in the former case

dcnJdlreci,l = 5.24 X lo* mdd M-l, whereas for the latter, the value is 350 x lo* mdd M-l). The hydrogen absorption values are also considerably less in the case of the inhibitor boosted with Cu2+.

Adsorption characteristics of the inhibitors. In order to test the adsorption characteristics of the studied inhibitors, their surface coverage at different concentrations was fitted in different adsorption isotherms. It is observed that the inhibitors obey the Langmuir adsorption isotherm equation. Values of log 19/( 1 - 13) vs log Care

1012

0.35

0.30

0.5

0

D. D. N. Singh, T. B. Singh and B. Gaur

_ Inhibitor: 300ppmHA + 1Oppm Cu*+

I I I I I I

0.1 0.2 0.3 0.4 0.5 0.6 0.7

FeCI, (M)

0.8

0.2

Fig. 9. The effect of ferric chloride on the corrosion rate and hydrogen absorption of steel in 3N HCI in the presence of optimized concentration of the inhibitor.

IO

HA 10% HCl _

Fig. IO. Plots of adsorption isotherm for HA in 3N HCI at different studied tcmperaturcs.

plotted in Figs 10-12. Here, 8 is the fraction of surface covered by the inhibitor, whereas (1 - 0) is the bare surface which acts as a site for corrosion reactions to proceed and C is the molar concentration of inhibitors.

Electrochemicalstudies. The polarization studies were performed in the presence of different combinations of the inhibitors in 3N HCI solution and the plots are shown in Figs 13 and 14. It is noted from these figures that the presence of HA in concentrated HCI solution has no greater appreciable effect on the anodic and


I 1 .^

IO-

log 0

Fig. 11. Plots of adsorption isotherm for cations having 300 ppm HA in 3N HCI at 23C

100 Temperature 40 C

300ppmHA

S L 2

5

Fig. 12. Plots of adsorption isotherm for cations having 300 ppm HA in 3N HCI at 40C

cathodic slopes than on the control plot. The presence of Cu2+, As3+, Sb3+ and Sn2+, however, has considerably affected the plots. Amongst the additives studied, the presence of CL?+ with HA has the maximum influence on the anodic curve, followed by Sb3+ and Sn+. The cathodic curve in the presence of Cu*+, however, is not changed at all. The cathodic plots in the presence of Sb3+, Sn*+ and As3 are considerably polarized.

Inhibition studies in dilute acid solution Weight-loss studies. Weight-loss corrosion data of steel exposed in N/200 HCl in

the presence of different inhibitors are shown in Table 1. Due to the very slow rate of


0

-075 -

10-3 10-2 10-l I IO 102 IO

c.d. (mA cm-*)

Fig. 13. Electrochemical polarization plots of steel in BN HCI in the prcscncc of 100 and 300 ppm HA with control (in blank).

-0.25 -

E

-0.50 -

9

4

-0.75 -

-l.Oo-

10-S 10-J 10-3 10-2 10-I I IO 102

c-d. (mA cm-)

Fig. 14. Electrochemical polarization plots of steel in 3N HCI in the prescncc of 300 ppm HA having different cations.


Table 1. The corrosion rate of mild steel and the zeta potential of the iron particles in N/200 HCI solution in the absence and presence of different

inhibitors at 29C

Solution Corrosion rate? Zeta potential S. No. composition (mdd x 102) (mv)

1 Blank (b) 10.6 -10 2 b + 100 HA ppm 9.6 -35 3 b + 300 HA ppm (c) 9.2 -40 4 c + 10 Cu2+ ppm 11.3 -16 5 c + 10 As+ ppm 7.2 -35 6 c + 10 Sn2+ ppm 16.6 -35 7 c + 10 Sb+ ppm 42.8 -35

:Derived from a 72 h exposure test.

attack of the electrolyte on the steel surface, the experiments were performed for an exposure period of 72 h. In contrast to its role in concentrated acid solution, HA exhibits very poor inhibition in this case. HA, in combinations of all the cations except As?, has a rather accelerating effect on the corrosion of steel. Amongst the cations studied, Sb+ has the highest rate of acceleration, followed by Sn2+ and cu2+.

Zetapotential measurement. The peculiar results observed for the performance of different inhibitors on the corrosion of steel in dilute HCI solution led to a study of their inhibitive nature in detail. The zeta potential studies, which are generally useful for study of the surface charges on the minerals suspended in a fluid, were performed for the particles of iron (iron powder of 200 mesh) suspended in N/200 HCI having different inhibitors. In these studies, since the experiments were performed within l- 2 min of exposure of the iron particle in acid solution and also due to an extremely slow rate of attack of electrolyte on the metal surface, the shapes of the iron particles were taken as unchanged during the experimentation. The measured values of zeta potential are summarized in Table 1. It is seen from this table that the values of zeta potential on the iron particle exposed to N/200 HCI solution (pH = 3.5) is negative (- 10 mV). A negative value of zeta potential is expected for this system owing to the strong adsorption of chloride ion in the ionic sphere of the particle in the solution. On the addition of 100 ppm HA in the bath, the zeta potential jumps to a more negative value (-35 mV). On increasing the concentration of HA (300 ppm), the value shifted towards the negative direction. The addition of Cu2+ with HA, however, brings down the potential to a lower negative value (-16 mV), very close to the blank solution where no inhibitor was present. A similar trend is noted for Sn2+ and Sb3+ also and the values are very close to that observed for Cu+. In the case of As3+, however, the particles retain their potential to a more negative value as noted for HA alone.

Electrochemical studies. The open circuit potential-time plot for the steel exposed in different compositions of inhibitors and in their absence are shown in Fig. 15. In all the cases, it is noted that the potentials shift in the active direction with the


2

Time (minj3 4 5

Fig. 15. Potential-time plots for steel in N/200 HCI in the presence of different compo sitions of inhibitors.

.-------- 3WppmHA+ 10ppmAs3+ -_- 300ppmHA + 1Oppm Cu2+ ._-_- 300ppmHA + 1Oppm Sb

-0.25 - . -- - 300ppmHA + 1Oppm Sn2+

- 100 and 300ppmHA

- - - - Control

-0.75 -

10-I 1 10

cd. (mA cm-)

Fig. 16. Electrochemical polarization behaviour of steel in N/200 HCI solution having different inhibitors.

passage of time. HA shows the highest negative value of stabilized potential followed by Cu2+, control and A?+.

The cathodic and anodic polarization curves in the presence of different compositions of the inhibitors are shown in Fig. 16. The plots for Sb3+ and Sn2+ are not


included here to avoid crowding. Their trends were also observed to be similar to Cu2+. In the presence of HA, the cathodic curves are polarized to a greater extent than the controlled one, especially near to the corrosion potential. The anodic curves, on the other hand, in the presence of HA, are depolarized near the corrosion potential. The overall corrosion, however, is found to be decreased although not to a considerable extent.

The addition of Cu2+ with HA accelerates the attack and is manifested with anodic as well as with cathodic curves. The presence of As3+, on the other hand, has a strong polarizing effect on anodic reactions.

DISCUSSION

An iron surface attains a positive charge in aqueous acid solutions as its potential of zero charge varies between -0.4 and -0.7 V (SCE).5*6 Owing to the strong anionic component of HCl, the double layer acquires the negative charge and cationic type of inhibitors are, therefore, found to provide good inhibition for iron in acid solutions.

HA forms a quaternary type of compound when added in the acid solution:

I N

/

\ CH2 CH,

\ \ I& N-CH2-NH

C/H, :.H2

+

[W

Due to the formation of a cationic surfactant in the solution, HA is expected to inhibit the corrosion cathodically. This is indeed reported by many other investigators.9--2

To understand the role of different inhibitors in inhibiting the corrosion of steel in HCl solution, the following scheme is considered for the adsorption of HA:

ElElElElEl- ooooo- +++++t---

protonized HA

chloride ion adsorbed

steel surface

Adsorbed chloride ion on the positively charged steel surface acts as a bridge between the metal surface and the electrolyte for the adsorption of the protonized HA molecule. The presence of a more densely populated interface with anionic species is expected to provide a higher degree of inhibition to HA according to the above scheme. This has indeed been observed in the study where an increase in the


-x- HAalone _ -.- - HA + NaCl

/ I I --- J___L__-l-_-L- 0 2.5 5.0 1.5 10.0

NaCl in 3N HCl (%)

I I I I I I 0.3 2.0 4.0 6.0 8.0 10.0 I

Concentration of HCI (N) .O

Fig. 17. The variation in the performance of inhibitors for steels at different conccn- trations of HCI and NaCI.

concentration of HCI enhances the inhibitive performance of HA (Fig. 17). Incor- poration of NaCl in 3N HCl having 300 ppm hexamine also enhances its performance. This theory is further supported by the observation on the performance of the inhibitor in extremely dilute HCl solution (N/200), where an almost negligible inhibitive effect has been noted (Table 1).

The enhanced inhibitive effect of HA in the presence of metal cations (in 3N HCI) can be explained by considering the ionic species formed in concentrated acid solutions. In strongly acidic chloride environments, the copper cation forms species of the type CuCl, . 2- Such types of anionic species are more surface active and can cover more surface area on the interface compared to the chloride ion alone. Such types of adsorbed species are also expected to facilitate the adsorption of protonized HA to a greater extent on the metal/electrolyte interface than the chloride ion. Though there is no reference available in the literature on the combined effect of metal cations with HA on the corrosion of steel, some authors have reported an improvement in the inhibitive performance of HA in combination with certain anions, such as Cl-, Br , I-, CNS and sulfonates, on the corrosion of steel in acidic solutions.6.7 The improved performance of HA in combination with Asa+ may be ascribed to the formation of anionic species AsO; in the acidic solution. This anion, like CuClzP, is expected to enhance the adsorption of HA+ at the interface of the corroding metal. Sb+ and Sn 2+ in the acid solution also enhance the protection properties of HA and their beneficial effects can also be explained as described above for Cu2+ and Asa+.

The adverse effect of the studied metal cations on the inhibitive performance of HA in dilute acid solution is quite an interesting phenomenon. HA has a negligible effect in diluted acid solution (Table 1). This is probably owing to the inability of HA to form cationic species in dilute acid solution. Due to the presence of a weak negatively charged interface, the HA molecule adsorbs on the metal-electrolyte interface though the lone pair of unshared electrons available at the nitrogen atom of


HA and, thus, increases the negative charge of the interface. This is also manifested by the zeta potential value. In the absence of any additive, the zeta potential value of the iron particle is -10 mV, which jumps to -35 mV on the addition of HA in the electrolyte. The addition of copper cation in the presence of HA replaces the latter from the interface and may deposit at the metal surface on cathodic sites. This brings down the zeta potential value (- 16 mV) of the particle. In the case of Sn2+ and Sb3+, no change in zeta potential takes place, indicating that they are not reduced at the metal surface and remain in the solution to act as strong depolarizers of the reaction. Unlike the other three cations, As3+ shows an inhibitive effect with HA in dilute acid solution. This is probably owing to the amphoteric nature of arsenious acid (HAs02) which provides AsO+ and AsO; in the acidic solutions. AsO+ is reduced at the cathode to deposit as a neutral arsenic atom on the surface:

AsO+ + 2Hf + 3e- + As + H,O

Deposited arsenic on the surface increases the cathodic overvoltage and provides the inhibition effect. Deposition of neutral arsenic metal on the surface is further manifested from the zeta potential value. The value of zeta potential of the iron particle exposed in solution having 300 ppm HA does not change after the addition of As3+ in the solution (in both cases the value is -35 mV). It is to be noted here that the inhibition caused due to the As3+ in dilute acid solution is solely due to the deposition of arsenic metal on the surface and HA has neither an additive nor synergistic effect on it.

Acknowledgemenr-The authors express their thanks to Prof. P. Ramchandra Rao, the Director, National Metallurgical Laboratory, for granting the permission to publish this work.

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