Liquid ammonia, stress corrosion cracking

Stress-corrosion cracking (Section 8.10) New metal/environment combinations which produce stress-corrosion cracking are continually being found. Combinations discovered in service in recent years include titanium in red fuming nitric acid carbon steel in liquid anhydrous ammonia and in... 

Lunde, L., and Nyborg, R., Stress corrosion cracking of different steels in liquid and vaporous ammonia. In Proceedings of Corrosion 87, San Francisco, 1987, paper 174, NACE, Houston (1987)... 

Materials of construction for ammonia are dependent on the operating temperature. Whilst mild steel may be used at ambient temperature, special costly steels are required at low temperatures to avoid embrittlement. Impurities in liquid ammonia such as air or carbon dioxide can cause stress corrosion cracking of mild steel. Ammonia is highly corrosive towards copper and zinc. Rubber lined steel construction is suitable for service at ambient temperature. 

One of the major problems encountered in the storage and transport of anhydrous liquid ammonia is the stress-corrosion-cracking (SCC) of carbon steel equipment. Cracks most often occur at the weld joints, where the leftover stress is at a maximum. The leftover stress is that which remains even after heat treatment. The hardness of the material and the presence of impurities and oxygenates in ammonia aggravate SCC88. 

FIGURE 20.37 Stress corrosion cracking in an unalloyed steel storage tank for liquid ammonia. 

WUde, B. F. (1981) Stress corrosion cracking of ASIM A517 steel in liquid ammonia Environmental factors. Corrosion, 37, No. 3. 

The liquid ammonia from the separator contains a small amount of dissolved gases. These are partly released by pressure reduction in a iet-down vessel normally to about 2.5 MPa. After the let-down vessel the ammonia is further flashed to almost atmospheric pressure before being sent to the ammonia storage tank. In order to reduce the risk of stress corrosion cracking in the ammonia storage tank, the ammonia product should contain minimum 0.2% of water. 

Ammonia and sulfides can also be produced from the decay of organic matter within the slime film, resulting in increased corrosion of some alloys [28], Sulfur-oxidizing organisms produce sulfuric acid from sulfur or other reduced sulfur species [2S]. The presence of anunonia is known to cause stress corrosion cracking of copper alloys, while sulfides may lead to accelerated attack on copper alloys and steel. The presence of the slime film on the metal surface can locally change the local environment at the liquid/metal interface such that the corrosion behavior of a metal can be considerably altered from one that nonnally displays low corrosion rates in seawater to conditions where corrosion is accelerated [6]. 

Water also inhibits the stress corrosion cracking of steel in ammonia, and titanium in methanol, as well as attack on titanium by dry chlorine. A trace of water (0.001%) in liquid hydrogen fluoride (HP) behaves as a passivating inhibitor for nickel. This is an extreme example of the importance of solvent-inhibitor interactions. The exact mechanism of inhibition by water in HF is unknown, but the passivating effect is similar to that observed on steel in the presence of chromates in aqueous solution. 

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