The influence of plasma nitriding on the electrochemical corrosion of some steels used in nuclear power plants.

Nemes, Toderita ; Bibu, Marius ; Petrescu, Valentin 等

This paper presents the results of experimental researches referring to the corrosion resistance of A541 class 6 steel with plasma nitriding treatment, steel used in the construction of auxiliary components for nuclear power plants. The polarization curves and Evans diagrams for electrode potential and for the intensity of the corrosion current of parts inserted into a 3% NaCl aqueous solution have been traced. The interpretation of the diagrams shows a good corrosion resistance of this steel, with a large passivation interval.

Key words: A541 steel, plasma nitriding, corrosion

1. INTRODUCTION

The corrosion in aqueous environments (condensed humidity coatings, atmosphere, natural waters, fluids, soil and chemical environments) is of electrochemical nature, a process in which the metal reacts with the electrolyte and oxidizes (loses electrons), while a reactant from the solution is being reduced (gains electrons). Anodic and cathodic processes are coupled and take place at the same speed and at a common potential, called mixed potential (Badea, 2000).

The electrode potential ([epsilon]) represents the difference of potential between the electrode and the electrolyte as a consequence of the exchange of electrical charges between them. It is determined with Nernst's equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where: e is the electrode potential; [[epsilon].sup.0] is the standard electrode potential, corresponding to the metal introduced in a solution containing an ion concentration equal with the unit; T is the absolute temperature of the solution; F is Faraday's constant and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is the concentration of metallic ions (Vermesan, 2005).

For the experimental measurement of the electrode potential, a combination of two singular electrodes, forming a galvanic cell, is used. The measured difference of potential is the driving force of the reaction, called electromotive force and it is the only unit that is measurable by experimental means.

The potential of the studied metal, measured in relation to the reference electrode, varies according to the intensity of the applied current. If the composition of the electrolyte or the surface of the metal remains unchanged, the value of the electromotive force is calculated using the formula (Tafel's law):

E = m + n log(i) (2)

The polarization curves are obtained either by varying the intensity and measuring the electrode's potential (intensiostatic curves), or by keeping a constant potential and measuring the intensity of the current (potentiostatic curves) or if the applied potential is varied progressively, potentiokinetic curves are obtained.

The Evans diagrams are obtained by drawing the potential curves in an anodic potential--lg(i) coordinates system. These diagrams offer information about the nature thermodynamics and kinetics of corrosion processes.

Based on these diagrams, considering the Tafel regime, the kinetic parameters of corrosion can be calculated: density of current ([i.sub.cor]), corrosion speed ([K.sub.g]) and the penetration index ([P.sub.min]).

[i.sub.cor] = [I.sub.cor]/S, [mA/[cm.sup.2]] (3)

[K.sub.g] = 0,373 M/Z [i.sub.cor], [g/[m.sup.2]h] (4)

[P.sub.min] = [K.sub.g] x 8,76/[[rho].sub.M], [mm/year] (5)

where:

S--the surface of the sample, in [cm.sup.2];

M--the atomic mass of the metal, in g/mol;

Z--the number of electrons that take place in the oxidation process of the metal;

[[rho].sub.M]--the density of the metal, in g/[cm.sup.3].

2. EXPERIMENTAL RESEARCH

2.1 Characteristics of steels used in nuclear power plants

The main characteristics necessary for special steels used for nuclear power plants equipment are mainly a high chemical passivity of the surfaces and a very stable structure that do not allow intercrystalline corrosion (Badea, 2000). The high hardness of these steels is obtained through carbides, nitrides or intermediary stages of these. For this purpose, steels are alloyed with chromium and nickel and to obtain some special properties they can also contain silicon, manganese, nitrogen, tungsten, molybdenum, vanadium, titanium, cobalt, selenium and niobium (Vermesan, 2005).

Samples used for experiments were made of special A541 class 6 steel with the chemical composition indicated in table 1. This steel type is used, among others, to create flanges for coupling terminal fittings of reactors in nuclear plants (Nemes et al., 1999).

The main mechanical characteristics of the samples in tempered state are: [R.sub.p0,2] = 728 N/[mm.sup.2]; [R.sub.m] = 804 N/[mm.sub.2]; A = 15.7%; Z = 41.3% and hardness 62 HRC (Deac & Nemes, 2007). The samples were subjected to a plasma nitriding treatment in dissociated ammonia with following parameters:

--nitriding temperature: 520[degrees]C;

--pressure: 2.5 torr;

--treatment time at 520[degrees]C: 10 hours.

Observations made on the microstructure after plasma nitriding emphasized a layer composed of an area of y-combinations and an area of a-diffusion with fine separations of chromium carbides (Nemes et al., 2008).

2.2 Experimental equipment

The scheme of the installation for determining polarization curves for electrochemical corrosion is presented in Fig. 1.

The reference electrode employed for this purpose is made of calomel (mercury/mercury chloride), consisting of a platinum wire submersed in mercury, which is in contact with a paste of calomel and potassium chloride, over which the potassium chloride solution is poured (saline bridge or junction liquid). The link between the junction liquid and the test solution is provided by a diaphragm (made of porous ceramic material). The electrolyte used was an aqueous solution of potassium chloride with a concentration of 3% and a temperature of 22[degrees]C. The universal polarograph OH-105 has a reference potential of -4V and the starting potential is 1.6V.

The applied potential [V], current intensity [mA] and anodic potential [V] were measured simultaneously. The values are presented in table 2.

2.3 Experimental results

Using the data gathered from the experiments, the polarization curve (Fig. 2.) and Evans diagram (Fig. 3.) for the A541 class 6 steel were traced.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. CONCLUSIONS

By studying the polarization curve and the Evans diagram, the following aspects can be emphasized:

1. in 3% NaCl solution, the growth of the applied potential is accompanied, at first, by a slow growth of the current intensity up to approximately zero, then it stabilizes for the +0,9 ... -0,6V interval, followed by a rapid growth of the intensity to [I.sub.max] = 1,22 mA ([i.sub.cor] = 0,44 mA/[cm.sup.2]). Then, a sudden drop in intensity down to around zero takes place, followed by the passivation of steel which is maintained on a broad interval, offering the steel a good electrochemical stability;

2. the Evans diagram reveals that the corrosion process has a mixed control, but the two reactions--anodic ant cathode--unfold slowly and with approximate equal speeds;

3. applying plasma nitriding to the A541 class 6 special steel provides it with a higher electrochemical stability in saline environments.

4. REFERENCES

Badea, T. (2000). Stiinta si Ingineria coroziunii (Science and Engineering of Corrosion), Ed. Academiei, Bucharest, Romania

Deac, C. & Nemes, T. (2007). Researches Referring to Plasma Nitriding of Some Alloyed Construction Steels, Annals of DAAAM 2007 & Proceedings of the 18th International DAAAM Symposium, Katalinic, B. (Ed.), pp. 217-218, Zadar, Croatia

Nemes, T., Petrescu, V. & Nascu, H. (1999). Determinarea curbelor de polarizare la coroziune electrochimica a unor oteluri nitrurate ionic utilizate la centralele nucleare (Determining the polarisation curves at electrochemical corrosion of ionitrided steels used in nuclear power plants), Acta Universitatis Cibiniensis, Vol. XXXIII, ISSN: 1221-4949, pp. 103-108

Nemes, T. et al. (2008). Study on the Improvement of the Anticorrosive Resistance of Steels Through Combined Galvanic and Heat Treatment, Coroziune si Protectie Anticoroziva, Vol. III, No. 4, pp. 5-8, Cluj-Napoca

Vermesan, H. (2005). Coroziunea (Corrosion), Ed. Risoprint, Cluj-Napoca

Tab. 1. Chemical composition of A541 class 6 steel

C Mn Si S Cr Ni Cu Mo

0,3 0,54 0,22 0,012 2,33 0,36 0,015 0,097

Tab. 2. Polarization parameter values for A541 steel in
3% NaCl solution

Anode
potential I i log(I)
(V) (mA) (mA/[cm.sup.2]) (mA)

1,538 -0,95 0,26389 -0,02228
1,301 -0,56 0,15556 -0,25181
1,061 -0,27 0,075 -0,56864
0,817 -0,14 0,03889 -0,85387
0,657 -0,1 0,02778 -1
0,418 -0,07 0,01944 -1,1549
0,175 -0,04 0,01111 -1,39794
-0,144 0 0 --
-0,388 0,055 0,01528 -1,25964
-0,627 0,18 0,05 -0,74473
-0,866 0,62 0,17222 -0,20761
-1,105 0,82 0,22778 -0,08619
-1,344 1,15 0,31944 0,607
-1,585 0,06 0,01667 -1,22185
-1,829 0,068 0,01889 -1,16794
-2,066 0,085 0,02361 -1,07058
-2,228 0,095 0,02639 -1,02228
-2,382 0,11 0,03056 -0,95861