Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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19 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 Information on corrosion control and prevention corrosion in the presence of sodium hydroxide (NaOH), potassium hydroxide (KOH), or other strong alkaline composi- tions at high temperatures. Amine SCC is a form of alkaline SCC that occurs under the combined action of tensile stress and corrosion in the presence of an aqueous alkanolamine liquid at various tempera- tures, and is predominately intergranu- lar. Amine corrosion and SCC are ty pi- cally a concern in amine treating and regeneration units, in which aqueous alkanolamine solutions are used to remove acid gases such as H 2 S and carbon dioxide (CO 2 ) from various gas or liquid hydrocarbon streams. This ty pe of crack- ing is most often found at or adjacent to CS weldments without a PW HT or in highly cold-worked components. Carbon- ate SCC is also a form of alkaline SCC that ty pically occurs close to or in the vicinity of welds; and some cracking has been reported up to 50 mm away from welds that have not received PW HT. The fabrication practices recom- mended to avoid SSC, HIC, and SOHIC in wet H 2 S services can vary with the sever- ity of the service. The severity can be categorized based primarily on charac- teristics of the liquid water present (con- tinuous, intermittent, condensing, only from excursions, etc.); levels of corrosives such as H 2 S, hydrogen cyanide (HCN), H 2 , ammonium hydrosulf ide (NH 4 HS), and CO 2 ; the pH of the aqueous phase; and, if the component also handles amine, the amine ty pe (monoethanolamine [MEA], diethanolamine [DEA], methyl diethanol- amine [MDEA], etc.) and whether the amine is lean or rich. Table 1 can be used as a guide for identif ying the category/ severity of service. These severity levels are similar to some of the options pro- vided in NACE Publication 8X194, 4 and there is a NACE committee currently working to achieve industry consensus on these categorizations. Once the severity of wet H 2 S service is identif ied, various control measures can be set to mitigate corrosion risks associ- ated with these corrosive services, as shown in Table 2. The f irst control mea- sure is to establish requirements— chemistry restrictions, mill heat treat- ments, or additional requisites for the materials' physical properties—for the raw materials selected for the various equipment components. For example, carbon equivalent (CE) has a direct rela- tionship to the hardness of the HAZ. Base metal microalloying elements, such as vanadium, niobium, titanium, and boron, can also increase H AZ hardness and reduce the tempering effect of PW HT on the H AZ. Heat treatments for the base metal, such as normalizing (N), quench- ing (Q), or tempering (T), increase resis- tance to crack grow th in severity condi- tions classif ied as moderate and high. Susceptibility to hydrogen blistering and HIC in high severity services is pri- marily related to the quality of the plate steel (i.e., the number, size, and shape of the discontinuities). In this regard, the sulfur content of the steel is a key mate- rial property. Reducing the sulfur content TABLE 1. Severity of Service Category 1 (Low Severity) Category 2 (Moderate Severity) Category 3 (High Severity) • H 2 S concentration is 50 to 2,000 ppmw in water phase, and • No known cyanide compounds or cyanide concentration is <20 ppmw, and • No previous experience of signi•cant blistering, HIC, or SOHIC • pH range is 4.5 to 7.6 • Most lean amine services remove H 2 S • H 2 S concentration is between 2,000 and 10,000 ppmw in water phase, or • Previous experience in this or similar service showed signi•cant blistering, HIC, or SOHIC problems • pH range is 4.5 to 7.6 • Most rich amine services remove H 2 S • H 2 S concentration is above 10,000 ppmw in water phase, or • Presence of HCN or other cyanide compounds (>20 ppmw in water phase), or • Acidic aqueous phase with pH value <4.5 or >7.6, or • History of high corrosion rate and signi•cant cracking and/or blistering TABLE 2. Raw Material Requirements Based on Severity of Service Category 1 (Low Severity) Category 2 (Moderate Severity) Category 3 (High Severity) • Material shall be fully kilned (except tubing) • Hardness <200 HBN (base metal) • Chemical restrictions for plate: Nb: 0.020% max. V: 0.020% max. Ti: 0.020% max. Nb + V: 0.030% max. • Carbon equivalent for plates: 0.43% max. for up to 50 mm and 0.45% max. for >50 mm • Bolting and nuts exposed to the process stream are low hardness, such as ASTM-193, Gr. B7M and ASTM-194, Gr. 2HM • All low severity requirements • Vacuum degassed plates • Steel shall be normalized, N + T or Q + T • Additional plate chemistry restriction: P: 0.010% max. • Additional forging chemistry restriction: C: 0.30% max. • Plates shall be 100% ultrasonic tested per ASTM A578, S1, Level C 2 Option 1: • All moderate severity requirements • Additional chemistry requirements: S: 0.002% max. V: 0.008% max. Nb: 0.01% max. Ti: 0.02% max. B: 0.0005% max. • Plates shall be HIC tested per NACE TM0284 3 in Test Solution A with CLR ≤ 5% Option 2: • CS with 3 mm stainless steel or high-alloy clad/ weld overlay (no special requirements are needed on the CS base metal) Continued on page 20

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