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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37 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 dissolution in water. NH 4 Cl is considered an acid salt, and corrosion inhibitors are not effective. 8,13 NH 4 Cl formation also leads to fouling and plugging, causing pressure drops and loss of throughput. Ionic equilibria modeling (IEM) has been successfully used to determine system pH and salt formation tendencies to help choose appropriate neutralizers that can target both acid corrosion and under - deposit corrosion. 3-4,12 Corrosion modeling to predict corrosion rates has also been undertaken by researchers and validated with lab data. 13 However, field application of models has been less effective due to the u n i q u e a n d c o m p l e x c h e m i s t r i e s i n affected systems. Also, it is very challeng- ing to detect chemical constituents in pro- cess streams in time to respond effectively. Tackling this corrosion mechanism from both process and materials perspec- t i v e s r e q u i r e s a m u l t i - p r o n g e d a p - proach. 5-10,12-13 Minimizing water entrain- ment and chloride sources in hydrocarbon streams, along with temperature control through better design, can optimize the process. However, if NH 4 Cl precipitation is unavoidable, then water washing is in- stalled at appropriate upstream injection points. Computational fluid dynamics have been utilized to optimize wash water injec- tion schedules. 9 In practice, the volume of water injected is always above what is needed to achieve water condensation at the injection point. If excess water is not made available, there can be situations where incomplete dissolution of the salt oc- curs. This could lead to the formation of di- lute, corrosive solutions downstream of in- jection points upon local salt redeposition. All commonly used materials are sus- ceptible as described in API 571, 16 with CS being the most susceptible and titanium (Ti) being most resistant. Within the poten- tial-pH regimes under which NH 4 Cl corro- sion operates, the Cl – ion has enough driv- ing force to prevent oxygen adsorption on the steel surface at an atomic level. 15 This interferes with the repassivating ability, especially for stainless steels (SS) and cor- rosion-resistant alloys (CRAs). Addition- ally, upgrading from CS to Alloy 825 or duplex SS can shift the damage mechanism from underdeposit corrosion to chloride stress corrosion cracking. 2,5 Depending on th e turnaround tim e and budget con- straints, decisions can be made on upgrad- ing to higher nickel alloys (or using a clad- ding thereof ), which can mitigate NH 4 Cl corrosion to a large extent, but does not imply immunity. In this article, case histories from two refineries describing the rapid, localized, and unpredictable nature of NH 4 Cl corro- sion are presented. Best practice mitigation strategies are proposed as well. Case Studies on NH 4 Cl Corrosion Case History 1— Overhead Piping Failure in Crude Tower at Refinery A Following a through wall failure of the overhead piping on a crude tower, a ther- modynamic survey of the process environ- ment, and a metallurgical analysis of a pipe ring collected from the failure location were carried out. The results indicated that corrosive salts forming in the system were directly responsible for equipment damage and eventual failure. A pipe ring cut from the CS overhead line immediately upstream of the failure was removed and analyzed (Figure 1). Cross sections were prepared for metallography, scanning electron microscopy (SEM), and en erg y di sp ersiv e x- ray sp e c tro m e tr y (EDS). Figures 2 and 3 show the localized nature of the pits. The intensity of the color in the EDS map represents the relative con- centration of elements within the pit (green = chlorides). In addition to the preferential attack along the pipe bottom and the pres- ence of chlorides in the scale, the damage profile was consistent with a flowing, cor- rosive media attacking the metal surface. The operating temperature at this location was too hot for water to condense, leaving only corrosive salts as possible aggressors. Ion chromatography p er form ed on deposits collected from various locations in the overhead line helped identify the anions and cations in the environment. The pH was between 3.2–3.5. Chlorides were predominant (10,000 to 40,000 ppm), while amine composition was primarily methyl- monoethanolamine (4,000 to 8,000 ppm) a n d m e t h y l d i e t h a n o l a m i n e ( M D E A ) (10,000 to 30,000 ppm). NH 3 (~200 ppm) was also present. The presence of acidic compon ents and chlorides in th e salt FIGURE 1 As-received picture showing the thinned area (red arrow) along the pipe bottom. FIGURE 2 SEM image of a pit on the pipe ID. FIGURE 3 EDS mapping of the same pit indicating the presence of iron chlorides.

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