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Marcelle de Waal, a senior consulting engineer at One Eighty Materials Engineering Solutions.
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 postweld 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 oxidizers 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 lowertemperature 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.
There are a vast number of relevant industry standards used in corrosion evaluation, typically incorporated during the design conception of the heat exchanger, on a case-by-case basis. The use of standards is not limited to only design but also incorporated in maintenance plans and
subsequent failure analysis testing when required. Due to the vast list of standards available, thorough research of the relevant standards are required to ensure systems such as heat exchangers remain operational for their intended purpose.
Addressing preventative measures, De Waal stressed the importance of not interfering with a system unless done by a qualified servicing agent who should know the system ' s specific requirements and limitations, guided by the heat exchanger’ s manufacturer
De Waal recommended utilising testing laboratories that are accredited( SANAS) for water analysis, evaluating the water chemistry and verifying their compliance with standards like SANS 241 or as specified by the manufacturer. He advised clients to provide the heat exchanger ' s manual, as it specifies crucial water quality parameters for benchmarking. One Eighty integrates such water analysis results with material analysis techniques to offer a holistic understanding of corrosion issues and identify the root cause of the problem.
De Waal highlighted a common challenge: clients often send water samples to water testing laboratories that test against drinking water standards, receiving feedback that the water is‘ within acceptable ranges’. However, potable water standards are often insufficient for the highly specific requirements of heat exchanger systems. " Your manual will have a very specific range," he stressed, pointing out that heat exchanger manufacturers specify much finer tolerances for certain physical characteristics or chemical properties than what typical drinking water regulations typically offer.
Regarding dissimilar metal corrosion, De Waal explained that the risk of galvanic corrosion depends on how far apart the metals are on the galvanic series chart, which indicates the galvanic potential difference. He noted that zinc, aluminium and magnesium are more anodic, while copper and its alloys are more cathodic. These metal alloys are inherently not corrosive by itself, but when coupled by an electrolyte( such as water), galvanic corrosion can occur. This happens through an electrochemical reaction, that results in the more anodic metal to corrode. The key is to select bimetallic combinations that are as close as possible on the galvanic series to minimize the galvanic potential difference that would induce corrosive attack. Alternatively, galvanic corrosion can also function as a type of protective system, whereby a low-cost material is‘ sacrificed’ to protect a more costly and inherently corrosive susceptible metal from failing. One such example, was citing zinc anodes protecting carbon steel ship hulls that enables the longevity of the carbon steel plates against sea water by using the relatively low cast zinc anode blocks as sacrificial material. For stainless steels systems that consist of similar alloy types, such as 304 or 316, can effectively be paired together to form a type of heat exchanger to minimise material cost, without having a significant galvanic potential difference.
Continued in Part 2 …
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