Cupronickel acid corrosion

Carbon-steel tubes and cast-iron shells are least expensive, but high-sulfur crude oils may justify the use of 5 per cent chromium or even 18-8-chrome nickel tubes" particularly in cracking service. Refer to Chap. 9, pages 276 to 284. For the acid corrosion produced by salt brine, admiralty metal or occasionally cupronickel tubes are justified. Ammonia is often used as a neutralizer for acid corrosion, and it attacks admiralty metal so rapidly that admiralty metal cannot be used for such a situation. The mechanical properties of this alloy are also impaired by temperatures above 500°F, although under favorable conditions it can be used up to tube-wall temperatures of 600 F. Recommendations are as follows ... 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Cupronickels (Ni-Cu) In this category the most common cupronickel alloys are the Monel 400 and Monel K-500. The Ni-Cu alloys differ from nickel 200 and 201 because their strength and hardness can be increased by age hardening. Ni-Cu alloys exhibit higher corrosion resistance than commercially pure nickel, especially to sulfuric and hydrofluoric acids, and chloride brines. Handling of waters, including seawater and brackish water, is the major application of these two alloys in the CPI (e.g., desaUnation plants). In addition, Monel 400 and K-500 are immune to chloride-ion stress-corrosion cracking, which is often considered in their selection. 

The final group of copper alloys are the copper-nickel (cupronickels) alloys. They exhibit the best resistance to corrosion, impingement, and SCC of all the copper alloys. They are among the best alloys for seawater service and are immune to season cracking. Dilute hydrochloric, phosphoric, and sulfuric acids can be handled. They are almost as resistant as Monel to caustic soda. 

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