ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2008, том 44, № 5, с. 541-548
ФИЗИКО-ХИМИЧЕСКИЕ ПРОБЛЕМЫ ^^^^^^^^^^ ЗАЩИТЫ МАТЕРИАЛОВ
УДК 541.138
ASSESSMENT OF INTERGRANULAR CORROSION OF HEAT TREATED AUSTENITIC STAINLESS STEEL (AISI 316L GRADE) BY ELECTRON MICROSCOPY AND ELECTROCHEMICAL TESTS
© 2008 r. A. Kriaa*, N. Hamdi**, H. Sidhom***
*Departement de Chimie, Ecole Superieure des Sciences et Techniques de Tunis Rue Taha Hussein- Montfleury Tunis, Tunisia.
** Unite de recherche sur les materiaux, Institut National deRrecherche Scientifique et Technique, Tunis,
BP 95-2050 Hammam lif, Tunisia ***Laboratoire de Mecanique et Materiaux de l'ESSTT, 5 avenue Taha Hussein 1008, Montfleury, Tunis, Tunisia Поступила в редакцию 23.06.2007
The resistance to intergranular corrosion (IGC) of austenitic stainless steel varies during the process of aging at temperatures between 500 and 700°C. This follows the well-known phenomena of precipitation of M23C6 chromium carbides and intermetallic phases (n, o, x). Consequently, this leads to significant Cr-depletion zones at grain boundaries responsible for material sensitization to iGc. The assessment of the sensitivity to IGC, from the Strauss test or equivalent tests, requires cutting a sample off the material, which can be harmful to the integrity of the structure in service. This assessment remains by its aspect, qualitative and little sensitive to the low widths of Cr-depletion accompanying the beginning of precipitation. The DL-EPR method (Double-Loop Electrochemical Potentiodynamic Reactivation test) is known to be a non-destructive and quantitative test method for detecting relatively mild degrees of sensitization in austenitic stainless steel. The current ratios Ir/Ia > 1% (sensitization criteria) and as consequent the electric charge ratios Qr/Qa > 1% of the degree of sensitization (DOS) to intergranular corrosion can be considered as good parameter values to differentiate materials with only difference in the DOS and to detect the fine precipitation responsible for depletion in elements of alloy. This criteria is also valid for the detection of desensitization during aging for longer periods of time.
PACS: 81.65.Kn; 81.65.Rv
1. INTRODUCTION
Austenitic stainless steels are susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). The basic cause of both these forms of corrosion is sensitization. Exposure to a temperatures in the range 500°C to 800°C during welding or service, leads to the precipitation of chromium rich carbides (M23C6) at the grain boundaries and the formation of chromium depletion regions adjacent to these carbides [1, 2]. In this context, Cihal et al. [3] reported that the susceptibility of these steels to intergranular and/or localized attack is often provoked by heat treatment which, in turn, influences the shape of the reactivation curve. Also, the effects of heat treatment, at (or inadvertent exposure to) temperatures within the so-called sensitization range (~450-900°C) have frequently been the subjects of EPR studies.
On the other hand, the characterization and the assessment of this susceptibility to IGC of austenitic stainless steel can also be determined by normalized classical tests (ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159, AFNOR A05-160) of Huey, Strauss and Streicher [2]. These tests are destructive, difficult to perform on site and require of samples that can be harmful to the integrity of materials during their service. For this reason, the French company CAFL developed [4], an electrochemi-
cal, non-destructive test commonly known as the EPR (Electrochemical Potentiokinetic Reactivation), with the intent to measure the sensitivity to intergranular corrosion of austenitic stainless steels. In this context, we must cite several investigations [1-3, 5-20] that have contributed to the development of the EPR method. Different versions of this technique exist today, with the most frequently used being the double sweep rate method (DL-EPR) [5, 9] due to its lower sensitivity at the surface of materials [12, 17, 18].
The high sensitivity of this technique in regard to the electrolyte composition and the experimental conditions imply good reliability of the method on numerous classes of austenitic stainless steels and nickel alloys. However, the high precision observed to detect small precipitation requires for each material, a specific study of electrolyte composition as temperature, potential range and sweep rate [8, 12-16]. The most investigations described in the literature emphasized the basis of the electrolyte composition to be mainly sulphuric acid (H2SO4) added with a depassivator as the KSCN [12, 13, 14] or the NH4SCN [8, 16] or the HCl [13] in the case of the modified EPR method.
In this paper, we have used the EPR method to detect Cr-depletion zones causing precipitation at grain
542 KRIAA ë flp.
Table 1. Chemical composition of the studied steel, wt %
C S P Si Mn Ni Cr Mo Ti Nb Cu N B
0.022 0.015 0.020 0.35 1.74 13.4 17.3 2.13 <0.005 <0.005 0.04 0.035 14
boundaries of M23C6 chromium carbides and intermetallic phases (n, o, %) during the aging of 316L steel. The efficacy of the method for the detection of the degree of sensitization (DOS) to IGC is discussed by combining the results of aged microstructure specimens, completed by Cr-deple-tion profiles, with the results of Strauss and EPR DL tests.
2. MATERIAL AND METHOD
2.1 Chemical Composition
The study was carried out on a sample of austenitic stainless steel of type 316 L. The chemical composition is given in Table 1.
This steel was subjected to heat treatments of aging at temperatures between 550 and 700°C for 1h to 80000 h.
2.2 Experimental 2.2.1 Microstructure Study
Thin foils were employed to identify the various secondary phases. A Philips TEM/STEM CM30 analytical microscope in STEM mode at 300 kV equipped with energy- dispersive-X-ray spectrometer was used for microscopy and microchemical analyses.
The evolution of the microstructure during specimens aging has been studied by observations on optical microscope and Scanning Transmission Electron Microscope
Ir/Ia = 0
Fig.1. EPR DL test.
(STEM). The phases present were identified by electronic diffraction on thin foils, using energy dispersive X-ray analysis (EDXA) and their chemical compositions by X-ray microanalysis [22]. The profiles of chromium concentration at grain boundaries area were also established by X-ray microanalysis using an electronic microscope in transmission at 300 KV.
Microchemical analysis along a line perpendicular to the grain boundary in the vicinity of M23C6 carbides was performed using a focused electron probe of approximately 12 nm in diameter. The measurements were made at the same distance from the nearest carbide. Then the minimum chromium concentration ([Cr] at (t,T) wt. %) of each chromium depleted zone was determined and as a consequence, the depletion width was measured at a critical chromium concentration (12-13 wt. %).
Quantitative analysis of the major secondary phases, chromium-rich M23C6 carbides along grain boundaries and also n and a phases was conducted on micrographs of thin foils as was described previously [22].
2.2.2 Corrosion Tests
The sensitization to the IGC, during aging of the steel 316L, was evaluated by the destructive test (Strauss) and non-destructive test (DL-EPR).
Strauss Test:
The pieces aged at temperature between 550°C and 700°C were machined to final size (170 x 10 x 3 mm). For every state of aging, a test piece was preserved as check sample and the two others were immersed during 24 h in sulfo-cupric boiling solution. The detection of the sensitization to the IGC was evaluated by the depth of intergranular penetration determined on micrographic section, i.e., after a cross-section of the sample, final polishing and observation on optical microscope of the sample profile [22].
EPR-DL Test:
As regards the EPR test equipment, the test assembly consisted of a Tacussel-type PJT 24-1 potentiostat/gal-vanostat, a servovit generator and a millivoltmeter. The curves were plotted on an SE-790 x-y recorder. The series of DL-EPR tests were conducted as was conducted by others [2, 16, 23, 24]. After establishing the Ecorr, the specimen was polarized from the initial potential, Ecorr = -450 mV in the cathodic region to an anodic potential of +250 mV/SCE in the passivity region. As soon as this potential was reached, the scanning direction was
I
Table 2. Operative conditions and electrolyte composition for the control of austenitic stainless steel
Grades Temperature, °C Electrolyte composition Potential range, mVvsSCE Scan rate, mV/s-1 References
304-304L 30 H2SO4 0.5M + KSCN 0.01M -400 to +300 1.67 [13]
Z2NCDU25-20.04 Super austenitic ambient H2SO4 33 % + 0.3 % HCl -30 to +560 0.5 [14]
304LN and 316LN ambient H2SO4 0.5M + NH4SCN 0.01M -600 to +200 1.67 [9] [13]
316L 30 H2SO4 6M + KSCN 0.005M - 1.67 [15]
304-316L 26 H2SO4 0.5M + KSCN 0.01M - 1.67 [1]
AISI316L 30 H2SO4 0.5M + KSCN 0.01M -450 to +250 1.67 [2]
AISI308L and AISI 304L 30 H2SO4 0.5M + KSCN 0.01M -500 to +300 back to -500 1.67 [21]
AISI304 30 H2SO4 0.5M + KSCN 0.01M -400 to +300 back to -400 1.67 [17]
304LN-316LN 25 H2SO4 0.5M + NH4SCN 0.01M +200 to Ecorr 1.66 [16]
316L ambient H2SO4 0.1M + NH4SCN 0.01M -400 to +300 back to -400 0.5 This work
reversed and the potential was decreased to the cathodic region (Figure 1).
The DOS was evaluated by the DL-EPR method [2, 24]. The peak reactivation current (Ir) and the peak activation current (Ia) were measured during the backward and the forward scans, respectively. The ratio (Ir/Ia in %) and Qr/Qa in %, where Qr is the reactivation surface charge density and Qa, the activation passivation surface charge density was then determined. The sensitization of aged specimens is evaluated by the ratio of current densities (Ir/Ia) and by the ratio of electric charges (Qr/Qa) expressed in %
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