Promoter solutions, stress corrosion cracking

Copper and its alloys are resistant to alkalies with the exception of ammonium hydroxide and cyanides. Ammonium ions promote stress-corrosion cracking of copper and its alloys. Ferric and stannic salts are aggressive towards copper alloys. Ammonia and cyanide ions form tetramine copper and tetracyano copper complexes in ammonia and cyanide solutions, respectively. 

Stress-Corrosion Cracking. The conditions for SCC to occur are (i) a crack-promoting environment (ii) the susceptibility of material to SCC (iii) tensile stresses must exceed the threshold value. SCC is distinguished by the fact that the stress corrosion faces suffer very low corrosion, even in solutions that cause some damage to the free surfaces. As an example is the SCC of stainless steel at 200°C in a caustic solution or in aerated chloride solution where almost no traces of dissolution are visible on the crack faces (Figure 6.53).84,95 For example, SCC of metals has been by far the most prevalent... 

The necessary conditions for the occurrence of SCC are (i) a crack-promoting environment (ii) the susceptibility of the material to SCC (iii) exceeding threshold value with regard to tensile stresses. One of the distinguishing features of SCC is that stress corrosion faces suffer very low corrosion even in solutions that damage free surfaces. 

---