Date / Time: TBD
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.
Keywords: residual elements, carbon steel, hydrofluoric acid, electrochemical corrosion
Date / Time: TBD
Wire ropes with a sheathed spiral strand are commonly used for mooring applications in offshore oil and gas production. Each strand comprises a bundle of galvanized steel wires with a blocking compound applied to the outer layer of the wire bundle to prevent seawater from contacting the internal strands of wire if there is a breach in the urethane cover. The blocking compound and a sacrificial zinc layer on each strand of wire are designed to protect the carbon steel.
Depending on the water depth where the wire rope is being used offshore, it can experience a wide range of temperatures—from 22°C at the surface to 4°C at the seabed. Corrosion behavior of wires at these temperatures is critical, in case of breach in the urethane cover.
Immersion and electrochemical corrosion testing was performed on subsea mooring line wire rope with and without blocking compound in synthetic seawater at 4°C, 13°C, and 22°C. Samples of galvanized wire with and without blocking compound at the lowest temperature (4°C) did not exhibit any iron corrosion products even after 160 days of exposure to synthetic seawater. The corrosion rate of galvanized steel with blocking compound remained less than 2 mpy, while that for galvanized steel without blocking compound remained less than 5 mpy.
Tests performed with galvanized wire with blocking compound at 13°C did not show any iron corrosion products after 104 days of exposure to synthetic seawater. Both low-temperature tests (4°C and 13°C) were terminated after it was concluded that, at lower temperatures, the blocking compound was providing good protection to the galvanized steel.
Key words: Wire rope, electrochemical corrosion, blocking compound