Anhydrous ammonia is liable to cause stress crack corrosion in cargo tanks and cooling systems constructed of carbon-manganese steel or nickel steel.
In order to minimize the risk of stress crack corrosion the following measures shall be taken:
(a) Where carbon-manganese steel is used, cargo tanks, pressure vessels of cargo refrigeration systems and cargo piping shall be constructed of fine-grained steel having a specified
minimum yield stress of not more than 355 N/mm². The actual yield stress shall not exceed 440 N/mm². In addition, one of the following construction or operational measures shall be taken:
.1 Material with a low tensile strength (Rm < 410 N/mm²) shall be used; or
.2 Cargo tanks, etc., shall undergo a post-weld heat treatment for the purpose of stress relieving; or
.3 The transport temperature shall preferably be maintained close to the evaporation temperature of the cargo of -33° C, but in no case above -20° C; or
.4 Ammonia shall contain not less than 0.1 % water, by mass.
(b) When carbon-manganese steel with yield stress values higher than those referred to in (a) above is used, the completed tanks, pipe sections, etc., shall undergo a postweld heat treatment for the purpose of stress relieving.
(c) Pressure vessels of the cargo refrigeration systems and the piping systems of the condenser of the cargo refrigeration system constructed of carbon-manganese steel or nickel steel
shall undergo a post-weld heat treatment for the purpose of stress relieving.
(d) The yield stress and the tensile strength of welding consumables may exceed only by the smallest value possible the corresponding values of the tank and piping material.
(e) Nickel steels containing more than 5 % nickel and carbon-manganese steel which are not in compliance with the requirements of (a) and (b) above may not be used for cargo tanks and piping systems intended for the transport of this substance.
(f) Nickel steels containing not more than 5 % nickel may be used if the transport temperature is within the limits referred to in (a) above.
(g) The concentration of oxygen dissolved in the ammonia shall not exceed the values given in the table below:
+---------------+---------+
| t in °C | O2 in % |
+---------------+---------+
| -30 and below | 0.90 |
| -20 | 0.50 |
| -10 | 0.28 |
| 0 | 0.16 |
| 10 | 0.10 |
| 20 | 0.05 |
| 30 | 0.03 |
+---------------+---------+
