CORROSION SCIENCE
Low-Temperature Stress Corrosion Cracking of Austenitic and Duplex Stainless Steels Under Chloride Deposits Tomas Prosek,‡,* Anne Le Gac,* Dominique Thierry,* Sandra Le Manchet,** Christian Lojewski,** Amelie Fanica,** Elisabeth Johansson,*** Carolina Canderyd,*** Francois Dupoiron,**** Tom Snauwaert,***** Fleur Maas,****** and Benny Droesbeke******
ABSTRACT
KEY WORDS: atmospheric corrosion, chloride deposits, localized corrosion, stainless steel, stress corrosion cracking
Corrosion performance of austenitic-ferritic (duplex) stainless steels UNS S32101, S32202, S32304, and S32205 and austenitic stainless steels UNS S30403 and S31603 was studied in the presence of chloride deposits simulating non-rinsing atmospheric conditions. The effect of temperature, relative humidity, concentration, and composition of the chloride deposits on the tendency for atmospheric, low-temperature, chloride-induced stress corrosion cracking (SCC), pitting, and selective corrosion was assessed using prestressed samples with a circular weld. In the presence of calcium chloride, SCC was observed at temperatures as low as 20°C and 30°C on the austenitic stainless steels UNS S30403 (Type 304L) and UNS S31603 (Type 316L), respectively. The only cases of SCC of tested duplex stainless steel grades were found at 70°C, which otherwise suffered mainly selective dissolution of the ferrite phase with one order lower depth of attack. The initiation of SCC and selective/pitting corrosion was governed by the equilibrium chloride concentration in a solution formed by contact with chloride-containing deposits and with air at a given relative humidity. Threshold levels of critical chloride concentrations, critical relative humidity in the presence of specific deposits, and maximum temperatures for safe applications of the studied grades were established.
Submitted for publication: January 30. 2014. Revised and accepted: May 28, 2014. Preprint available online: June 16, 2014, doi: http://dx.doi.org/10.5006/1242. ‡ Corresponding author. E-mail:
[email protected]. * Institut de la Corrosion/French Corrosion Institute, France. ** Industeel Creusot ArcelorMittal Group, France . *** Outokumpu Stainless, Sweden. **** Total Petrochemicals, France. ***** Stolt Tankers, The Netherlands. ****** Belgisch Instituut voor lastechniek, Belgium.
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INTRODUCTION Stainless steel materials provide excellent service when selected and applied properly. A sufficient knowledge base is generally available for material selection for immersion conditions. This is not true for applications of stainless steels under atmospheric weathering conditions. Several cases of ceiling collapse and other component failures have shown that austenitic stainless steels are prone to stress corrosion cracking (SCC) under specific atmospheric conditions characterized by the spontaneous formation of concentrated chloride solutions under highly soluble chloride deposits, even at room or only slightly elevated temperatures. This was observed in indoor swimming pools;1-10 for outdoor climbing anchors;11-12 under evaporative conditions in oil and gas production, storage, and processing; and in other specific cases.13-14 The tendency of austenitic stainless steels to suffer atmospheric SCC in the presence of different chloride deposits has been studied by several research groups.5-6,15-27 It increases with increasing temperature, decreasing alloying level, and decreasing relative humidity (RH), for RH values above the deliquescence point of a given salt. It has been proposed that the initiation of this type of SCC is governed by the equilibrium chloride concentration in the surface electrolyte formed as a result of the interaction of chloride salt and water vapor in air.22
ISSN 0010-9312 (print), 1938-159X (online) 14/0000171/$5.00+$0.50/0 © 2014, NACE International
CORROSION—OCTOBER 2014
CORROSION SCIENCE
TABLE 1 Identification and Basic Characteristics of Tested Stainless Steels Surface Stainless Steel Grade Sample Thickness Roughness Specimen ID Trade Name UNS EN Type(A) (mm) Ra (μm) 2101/HRC 2202/HRP 2304/HRC 2304/HRP 2205/HRP 304L/HRC 316L/HRC (A)
LDX 2101 UR2202 2304 UR2304 UR2205 304L 316L
S32101 S32202 S32304 S32304 S32205 S30403 S31603
1.4162 1.4062 1.4362 1.4362 1.4462 1.4306 1.4404
HRC HRP HRC HRP HRP HRC HRC
6.0 6.0 6.0 8.4 8.3 6.0 6.0
3.5 4.4 4.2 3.8 4.2 4.7 5.1
HRC: hot-rolled coil; HRP: hot rolled plate.
TABLE 2 Chemical Composition, Structure, and Corrosion Resistance of Tested Stainless Steels Ferrite Austenite Grain Chemical Composition (wt%) Content Spacing Size CPT(B) Material C Si Mn P Cr Ni Mo Cu N (%) (μm) (μm) PRE(A) (°C) 2101/HRC 0.018 0.7 5.0 0.02 21.4 1.6 0.2 0.2 0.22 2202/HRP 0.017 0.4 1.3 0.02 23.0 2.5 0.3 — 0.21 2304/HRC 0.015 0.4 1.5 0.03 23.2 4.8 0.4 0.3 0.12 2304/HRP 0.018 0.4 1.3 0.03 22.7 4.3 0.3 — 0.15 2205/HRP 0.017 0.3 1.8 0.03 22.2 4.9 2.6 — 0.16 304L/HRC 0.019 0.3 1.6 0.03 18.2 8.1 — 0.5 0.07 316L/HRC 0.022 0.5 1.3 0.03 16.9 10.1 2.1 0.4 0.05 (A) (B)
50±2 53±6 49±2 46±2 53±2 — —
5.4 6.1 3.2 6.2 5.8 — —
— — — — — 16 23
26 18 27 37 26 33 26 30 33 49 19
