CS260662B1 - A method of comprehensive assessment of susceptibility of stainless steels to intergranular corrosion - Google Patents

Abstract

The solution concerns a method of comprehensive evaluation of the susceptibility of stainless steels and alloys to intergranular corrosion based on electrochemical potential measurements, where, during gradual potentiokinetic and potentiostatic polarization, changes in current density and electric charge are evaluated in the tested sample during the transition from the active to the passive state and vice versa. After potentiostatic polarization, the exposed sample is bent and the occurrence of cracks at the bend is evaluated.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for complex evaluation of susceptibility of stainless steels to intergranular corrosion.

In particular, immersion tests are known to monitor the susceptibility of stainless steels and alloys to local types of corrosion, which are both time-consuming, enorgotics and material-intensive. Potentiokinetic procedures, such as a method for assessing the properties of passive layers on metallic materials, are particularly known The principle of this method consists in forming a passive layer on a sample of metal material after preparation of a standard surface under predetermined conditions, and then subjecting the passive layer and the metal on which it has been formed to a passive layer. Reactivation anodic dissolution at decreasing potential and evaluation of total corrosion delay is also known. Further, it is known to evaluate structural changes of steels, especially those which increase the susceptibility of these steels to corrosion due to their heat and technological processing. AO 215 382, according to which steel samples in a suitably selected electrolyte are determined in both directions of the variation in the potential of the individual current density and their corresponding potentials for the activation and reactivation areas, and from these the structural changes are determined. they do not give unambiguous results, especially in steels and alloys with higher passive stability, for example in molybdenum-added steels. These known methods further improve the method of complex evaluation of the susceptibility of stainless steels and alloys to intergranular corrosion based on electrochemical potential polarization measurements, where gradual potentiokinetic and potentiostatic polarization assesses changes in current density and electric charge during transition from active to passive state according to the invention. The principle of the invention is that the steel of interest is treated in the region

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- 2 active peak potential + 0,05 Vg ^ g + 75 mV, detected potentiokineticly, then the following potentiostatic polarization reveals a change in current density over time, after which the occurrence of cracks is evaluated bilaterally by bending the sample ·

The occurrence of cracks in tensile stressed areas, as well as the increasing tendency of current density during potentiostatic polarization or its significantly higher value, or higher value of electric charge compared to equally exposed sample after solution annealing, is evidence of carbide precipitation and susceptibility to intergranular corrosion.

The process according to the invention is independent of one another in evaluating its result. For example, if unambiguous changes due to precipitation of carbides are shown on the potentiokinetic curve, no further potentiostatic polarization is required and the sample can be evaluated as susceptible to intergranular corrosion. If the potentiokinetic part of the evaluation does not show the susceptibility of the steel to the intergranular attack, a potentiostatic evaluation should be performed with the following bending of the sample. Depending on the type of steel or alloy to be tested, the aggressiveness of the electrolyte is selected, with the possible addition of an activating agent.

The method according to the invention has been defined by the potential for achieving optimal sensitivity for demonstrating precipitation processes in the steel of interest by a further subsequent quantitative potentiostatic measurement with bilateral sample bending control.

The evaluation method according to the invention is less time-consuming and energy-intensive than the tests used up to now. Significant reduction of test specimen and choice of shape allows evaluation of surfaces with different orientation to the original forming direction · The invention allows evaluation of steels and alloys with high stability of passive state ·

The invention is illustrated in more detail by means of the accompanying examples, describing 3,260,662 the possibility of a single or complex sequential evaluation, and of drawings or graphs, who in FIG.

Fig. 2 shows the potency-kinetics curve from oeoli lCr1SnilO (Ti) annealing 650 ° C / 60 min;

Figure 3 shows the progression of potentiokinetic curves between the potentials of +0.2 to -0.1 Vg _{EE of the} tested steel 1Cr18NilO (Ti) and 08Cr18Nil2Mo2 with different dwell at 650 ° C;

Fig. 4 recording constant current density variations · lCr18NilO (Ti) steel with different delay at annealing temperature and Fig. 5 recording constant current density variations ·

Steel 09Crl8Nil2Mn2 with different delay at annealing temperature ·

Example 1

Potoneiokinetic measurement of manganochrome austenitic o r type 04Mn24Cr13 with low passive state stability.

The measurement was carried out in the potential range of -0.7 to +0.2 Vg _EE in 0.5 mol.l “^ sulfuric acid at a potential change rate of 1 V in 6.6 minutes · The steel before the test was thermally affected at 650 ° C for 60 minutes and in its austenitic structure were carbide precipitates hg ^ Cg ·

The recorded potentiostatic curve, shown in Figure 1, shows a characteristic second active peak between the potentials of 0.0 to 0.1. This peak is related to the high proportion of manganese in the complex carbide Μ ^ Οθ, ρίίρ. in the reduced chromium content of this carbide and clearly signals the ongoing corrosion processes at the grain boundaries. Steel is resistant to intergranular attack ·

· The main active peak is shifted more to the left compared to nickel-containing steels and does not overlap the second active peak between the potentials of 0.0 to +0 in the transition from active to passive state, 1 ^ * SCE ^Druil characterized peak is caused by the corrosion and breakdown of carbide, their surroundings and corresponds extending intergranular corrosion.

Example 2

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Potentiokinetic measurement of chromium-nickel and chromium-molybdenum steels.

It is necessary to roll a more aggressive environment for chromiumlevel and chromium nickel molybdenum steels whose corrosion resistance increases with increasing content of alloying elements. sodium sulphate in a quantity of 2 g n with 100 ml of sulfuric acid solution and potassium rhodanide.

Since the increased nickel content shifts the potential of steel activity to higher values, the intergranular attack changes between 0.0 and +0.1 Vg / g overlap. yeah need to adjust the measurement procedure as follows:

After recording the uplink curve from -0.7 to +0.2 V _geE as in Example 1, it is necessary to change the direction of the potential increase and record part of the potentiokinetic curve continuously from +0.2 to min. —0.1 Vggg - at decreasing potential. This implies a partial passivation of the austenitic mass and a certain delay before reactivation. In this delay between the potentials +0,1 to 0,0 V _gC £ it is possible to record the secondary current density increase, caused by the causes leading to intergranular corrosion. A graphical record of the curve is shown in Figure 2. On the basis of this second active peak, the test material can be classified as non-crystalline-resistant against intergranular corrosion.

The limiting factor of this test is the sensitivity of the device at a very small procipitse range, or a significant improvement in corrosion resistance by alloying, for example, molybdenum. From Figure 3, which shows changes in potentiokinetic curves at the evaluated sites due to thermal sensitizing annealing, it can be seen that the characteristic change in molybdenum steel occurred after a longer sensitization period. In this case, the measurement should be supplemented by the potentiostatic procedure of Example 3.

- 5 Example 3

Potentiostatic procedure, _{2eo M2} £ evaluation of chromium carbides occurrence and susceptibility to intergranular corrosion can be used potentiostatic polarization at constant potential ·

The shape of the test specimen for this test was chosen to be suitable for the final evaluation bend made in two different directions on the original forming axis. The sample was first exposed according to example 2. Then the +0.05 'potential was placed on it for 30 to 60 minutes · During this delay the current density and the charge passed were registered ·

Based on these registered results, a first evaluation can be made because the high or increasing current density and charge passed have samples resistant to intergranular corrosion · A typical example of these changes is shown in Figures 4 and 5 · *

After completion of potentiostatic polarization for 60 minutes, the sample was evaluated by bending, and the occurrence of cracks in tensile stressed areas was monitored. These cracks are evidence of the resistance of the test material to intergranular corrosion.

The invention can be used for a comprehensive evaluation of the susceptibility of stainless steels and alloys to intergranular corrosion attack, in particular for power and chemical engineering.

Claims (1)

Method of complex evaluation of stainless steel susceptibility to intergranular corrosion based on electrochemical potential polarization measurements, where gradual potentiokinetic and potentiostatic polarization assesses changes in current density and electric charge during transition from active to passive state and vice versa, characterized by the steel of interest is applied in the area of the active peak potential of +0.05 Vg ^ _E + ⁷⁵ mV, detected potentiokineticly, at the following time: potentiostatic polarization so sgišiujo change in current density over time, whereupon the occurrence of cracks in the bilaterally bent sample is evaluated ·