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Electrochemical Corrosion Behavior of Carbon Steel With and Without Residual Elements

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Residual elements (RE) in carbon steel, not specifically included in the specified steel, appear to influence the corrosion rate under certain conditions, especially in services involving hydrofluoric acid (HF). The relative proportions of RE, specifically %C, %Ni, %Cu, and %Cr in carbon steel base and weld metals used in refineries, especially in alkylation processes with HF as the catalyst, significantly impact corrosion behavior. Studies described in the literature show corrosion damage with high RE (Cu + Ni + Cr >0.20) components as compared to low RE (Cu + Ni + Cr <0.20) components.  

In this study, electrochemical corrosion testing was performed on a 3-inch pipe elbow section with high REs that had developed a through-wall leak in service. Test results were compared to those obtained on a similar pipe elbow section with lower REs. The samples were exposed to 50% HF at room temperature and at 65°C. Linear polarization resistance (LPR) corrosion rates were measured at both temperatures. Potentiodynamic (PD) polarization scans were performed on samples of low and high RE steel exposed to 50% HF at room temperature.  

Test results indicated that LPR corrosion rates were higher for the high RE carbon steel samples than for low RE carbon steel samples at both temperatures. PD scans showed that the critical current densities were higher for high RE steel than for low RE steel.  

Product Number: 51320-14245-SG
Author: Suresh Divi, Sri Krishna Chimbli
Publication Date: 2020
Industry: Refinery/Gas Processing
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97513 EFFECTS OF RESIDUAL ELEMENTS ON THE CORROSION RESISTANCE OF STEELS IN HF

Product Number: 51300-97513-SG
ISBN: 97513 1997 CP
Author: G. Chirinos, S. Turgoose, R.C. Newman
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Picture for Importance of HF Acid Concentration on the Effect of Residual Elements
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Importance of HF Acid Concentration on the Effect of Residual Elements

Product Number: 51319-13428-SG
Author: Huang Lin
Publication Date: 2019
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Past HF alkylation operation experience and several industry research projects have reported that the carbon steel corrosion rate in HF acid is strongly affected by the residual elements (RE) content (Cu Ni and Cr). Low RE carbon steel has been recommended for new construction of carbon steel equipment and piping in main acid service trace acid service and dilute HF service in API 751.However one aspect that is often overlooked is the effect of the local concentration of HF acid on low RE steel. At low concentrations low RE carbon steel can be subject to preferential corrosion when it is coupled with high RE carbon steel. Normally only the bulk HF concentration in a process stream is available. It is challenging to determine the actual local HF acid concentration in a practical manner. This paper discusses the possible variations of HF acid concentration in typical HF alkylation processing systems and the corresponding effects of RE on HF corrosion. Possible electrochemical mechanisms of RE on HF corrosion are also studied. Several RE related preferential HF corrosion cases reported in the past have also been reevaluated from the HF acid concentration point of view. Evaluation of actual local HF acid concentration is strongly recommended when low RE carbon steel is applied. This can be more critical to the modified HF alkylation process which utilizes modified HF with a lower volatility and can have different HF concentration in the fractionation section.
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Effect of Residual Copper, Nickel, and Chromium on the Corrosion Resistance of Carbon Steel in Hydrofluoric Acid Alkylation Services

Product Number: 51393-93623-SG
Author: H.H. Hashim, W.L. Valerioti,
Publication Date: 1993
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Carbon steel is the main construction material in HYDROFLUORIC ACID (HF) alkylation units. Carbon Steel has good corrosion resistance to anhydrous HF (AHF) below 160 degrees fahrenheit (71 C). The corrosion resistance is due to the formation of an inorganic iron fluoride scale on the carbon steel surface that protects the steel from futher corrosion. The presence of an adherent and continuous scale is essential in keeping the corrosion rate at a minimum. 
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