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CHEMICAL TREATMENT FIGURE 5
Inhibition of Pitting on a Type 316L Stainless Steel Acetic Acid Tank
Salt from moist sea air depositing in the tank when it was left open after con- struction, and salt tracked into the tank on workmen's shoes, apparently in- creased the chloride content in the acid to the 740 ppm level observed in the green acid.
Corrosion Control The electrochemical test results (Fig-
Chrono-potentiometric curves obtained at different galvanostatic pulses for the Type 316L weld metal in the green acid blend.
potential of 100 mV below the recorded protection potential is achieved. Taking a value of –1 µA/cm2
as a target CD
for the impressed current CP system, a bare 50,000-gal (189-m3
) tank will require
0.33 A of current. It is common industry practice to use
AISI Type 316L SS for the storage and transport of HAc.1
Typically, for AISI
Type 316L SS in HAc service, especially when hot, the maximum allowable Cl– contamination appears to be <20 ppm.2 The tank plates in the failed area were
AISI Type 316L SS, which met the alloy composition limits of ASTM A2403
for
SS plates except for the nickel content. The nickel content is below the 10 to 14% alloy range by an inconsequential amount, which is within the accuracy of the OES instrument. The microstructure seen on Figure 2
would normally be considered acceptable for service. In particular, the microstruc- ture is not sensitized in the classical sense, because the grain boundaries are not heavily and completely ditched by the electrolytic oxalic acid etchant. It is clear, however, that the HAZ pitting is initiat- ing in an intergranular fashion (Figure 2). Apparently, the weld procedure is re- sponsible for a low level of grain bound- ary sensitization, which would normally not lead to problems in service.
50 MATERIALS PERFORMANCE April 2012
The external welds of the stiffening beams were apparently made with a root pass heat input of ~6 kJ/mm, whereas 1.5 kJ/mm is more common. The high heat input could be one reason for the unusual occurrence of pitting corrosion on the tank walls opposite the external stiffening beams. The most common cause of pitting corrosion on AISI Type 316L SS in acidic solutions is contamination by chlorides.4-6
The chlorides disrupt the
chromium oxide surface film, which protects SS, and can lead to microenvi- ronments in the pits, which become au- tocatalytic and propagate at high rates. It is clear that the green acid is contami- nated with 740 ppm chlorides (Table 2). Consequently, it would be expected to be a severe pitting environment for Type 316L SS.
Sources of Chlorides The inhibitor mixture is most likely
the primary source of the chloride con- tamination. The level of chlorides sug- gests that the inhibitor was made with an organic chloride intermediate. When diluted to 5% by blending with the HAc and water, the inhibitor would contribute several hundred ppm of chloride as con- ÅZUML Ja \PM IVITa[Q[ WN \PM ZML IKQL which showed 585 ppm Cl–
ion (Table 2).
ure 5) indicate that CP would be a viable method for controlling the pitting corro- sion in these tanks. A small impressed current CP system using inert anodes would be best. 6Q\ZWOMV JTIVSM\QVO _W]TL JM JMVMÅ-
cial in reducing chloride intrusion from [IT\ IQZ IN\MZ KWV[\Z]K\QWV IVL JMNWZM ÅTT- ing. Nitrogen blanketing in service would reduce the oxygen level, which would strongly reduce the metal's tendency to- ward pitting. The most effective measure to prevent recurrence of this pitting problem is to remove the inhibitor mixture. Replacing the 5% inhibitor mixture with more water would reduce the freezing point of the JTMVL
z The presence of 5% (v/v) corrosion inhibitor induces stable pitting nu- cleation because of the presence of 700 ppm of Cl–
in the inhibitor.
The situation with the fabrication welds indicates an accelerated pit propagation rate under the same conditions.
z The spent acid solutions, typical of long storage periods, is more aggres- sive in pitting propagation kinetics
NACE International, Vol. 51, No. 4