www. refrigerationandaircon. co. za RACA Journal I October 2025 43
Technical embrittlement. Additionally, elevated iron content dramatically increases electrical conductivity, accelerating galvanic corrosion in bimetallic systems like carbon steel shells with copper tubing, creating a self-destructive, negative feedback loop.
De Waal detailed the potential risks associated with various out-of-specifications parameters:
• pH: pitting and stress corrosion cracking in stainless steels.
• Electrical conductivity: increased potential for galvanic corrosion in bimetallic systems.
• Chlorides: localised pitting and crevice corrosion, especially around welded components of stainless steel alloys.
• Sulphur: dealloying of copper materials and increased risk of stress corrosion cracking in both copper and stainless steels alloys.
• Ammonia and nitrates: weakening of the passive oxide layer in stainless steels and stress corrosion cracking or dealloying of brass and copper alloys.
• Water hardness: excessive scale formation, leading to crevice corrosion and loss of thermal efficiency.
• Dissolved oxygen: provides oxygenated environment that promotes the formation of oxides( uniform- and localised-pitting corrosion) and promote an ideal micro-organism ecosystem that can lead to microbiologically-induced corrosion.
• Microbiological saturated water supplies: excessive microbial attack( biofouling) and scale formation.
De Waal presented a case study involving ammonia-induced stress corrosion cracking( SCC) in copper alloys. He showed microscopic images revealing‘ lightning crackling’ effects through the copper microstructure, indicating the devastating impact of ammonia. Ammonia can enter the system through refrigerant gas leaks or even through the introduction of cleaning agents, such as bleach, added to the water supply in an attempt to curb the growth of microbiological organisms, which contains ammonia. He stressed that proper water treatment and avoiding unauthorised chemical additions are paramount for preventing such severe corrosion.
Subtle signs, like a subtle to rapid drop in gas pressure without clear visible pipe damage, can indicate a high likelihood of stresscorrosion cracking that isn’ t always presented with evidence of typical surface oxide or rust. De Waal recounted an experience with a freon gas system in a factory where long sections of stainless steel 304L pipes suspended along the various parts of the factory were exposed to an environment rich in calcium carbonate( limestone) and excessive moisture due to condensation and rain. The combination of environmental exposure and physical inherent stress from the pipe ' s own suspended weight provided the necessary conditions to induce stress corrosion cracking. These cracks could only be visibly identified by the subtle gas-discharge and through non-destructive inspection tools, such as dye-penetrant testing. Stress corrosion cracking is a complex phenomenon requiring both a corrosive medium( like chlorides for stainless steel or ammonia for copper / brass) and a form of applied / inherent stress within the system.
De Waal further broadened the discussion to other common forms of corrosion:
• Sulphide pitting: occurs when sulphur compounds in water are significantly elevated, often in conjunction with microbiologically induced corrosion.
• Erosion: caused by turbulence and high velocities in the fluid flow, which can slowly erode the wall thickness of pipes, especially at bends. Proper thickness, diameter and flow velocity are crucial for prevention.
• Dealloying: as seen in copper alloys, this process involves the selective removal of specific elements from the microstructure, leaving behind a brittle and weak‘ skeleton’ that eventually leads to either corrosion, mechanical failure, or worse-case scenario, both.
Focusing on stainless steels, de Waal reiterated that they are susceptible to stress corrosion cracking, often induced by chlorides. Other common corrosion types in stainless steels include:
• Pitting: localised corrosion that creates small pin holes or perforations in the material.
• Crevice corrosion: typically occurs in confined spaces, like certain pipe joints or low flow velocity cavities within the system, where stagnant conditions and chemical differences can accelerate corrosion within these localised zones. He highlighted the importance of proper welding procedures and post-weld cleaning to prevent these hot spots to be excessively susceptible to corrosive attack of this nature.
• Microbiologically Induced Corrosion( MIC) or biofouling: caused by bacteria, such as sulphur-reducing bacteria, can clog systems, reduce efficiency and degrade materials. While most systems often use treated water free of bacteria, or add oxidisers to kill bacteria, untreated water sources rampant with microbial life can increase the likelihood of microbiological induced corrosive attack.
For carbon steels, the most common corrosion mechanism is oxidation, which results in the familiar red-brown rust to form uniformly across the materials surface. This is more evident in systems with elevated temperatures, as higher temperatures accelerate the corrosion rate. In lower-temperature systems, corrosion typically occurs at a slower rate and becomes only noticeable after multiple service maintenance periods, where backwashes are observed to have become noticeably red and cloudy in appearance. He stressed the importance of considering the shell side of carbon steel heat exchangers, as similar degradation can also occur externally if the protective paint coating system is not carefully maintained.
www. refrigerationandaircon. co. za RACA Journal I October 2025 43