In a SAIRAC Cape Town Centre TechTalk on 13 May, Marcelle de Waal, senior consulting engineer at One Eighty Materials Engineering Solutions, highlighted the critical role of water quality in corrosion prevention and heat exchanger longevity. This is Part 1 of a two-part series.
ASSOCIATIONS
Mastering heat exchanger longevity through a focus on water chemistry Part 1
By Eamonn Ryan
In a SAIRAC Cape Town Centre TechTalk on 13 May, Marcelle de Waal, senior consulting engineer at One Eighty Materials Engineering Solutions, highlighted the critical role of water quality in corrosion prevention and heat exchanger longevity. This is Part 1 of a two-part series.
De Waal provided an overview of common industrial heat exchanger types, such as shell-and-tube and plate designs, made from both ferrous and non-ferrous materials. These are widely used in applications from boilers to chillers. The core of De Waal’ s talk revolved around water chemistry as a primary driver of corrosion, particularly on the water side. He identified several key water parameters contributing to corrosion:
• High or low pH levels accelerate chemical reactions with metals
• Elevated electrical conductivity creates runaway reactions, especially in bimetallic systems prone to galvanic corrosion
• Ammonia can cause severe corrosion in copper alloys, leading to dealloying and embrittlement
• High calcium content promotes scale formation and crevice corrosion, reducing thermal efficiency
• Chlorides are aggressive towards stainless steels, causing pitting and stress corrosion cracking
• Iron from degrading ferrous components increases conductivity, accelerating corrosion
• Copper and sulphates are linked to microbial-induced corrosion
• Turbidity introduces suspended solids, leading to scale, blockages and corrosion
Heat exchanger manufacturers design their equipment to operate within a narrow pH range, often between 6.8 and 8.9. De Waal referenced the SANS 241 code for potable water, which outlines quality parameters and aligns closely with manufacturer standards.
The most aggressive parameters vary by material. For stainless steel, chlorides break down the protective oxide layer, enabling corrosion. In copper systems, ammonia causes dealloying and embrittlement. Elevated iron increases conductivity, accelerating galvanic corrosion in bimetallic systems.
De Waal outlined risks associated with various parameters:
• pH: Pitting and stress corrosion cracking in stainless steel
• Conductivity: Accelerated galvanic corrosion
• Chlorides: Pitting, especially around welds
• Sulphur: Dealloying of copper; stress cracking in copper and stainless steel
• Ammonia / nitrates: Weakening of oxide layers; dealloying of brass and copper
• Hardness: Scale formation, crevice corrosion, thermal inefficiency
• Dissolved oxygen: Promotes oxide formation and microbiologically-induced corrosion
• Microbial activity: Biofouling and scale
A case study showed ammonia-induced stress corrosion cracking( SCC) in copper alloys. Microscopic images showed‘ lightning crackling’ effects. Ammonia can enter via refrigerant leaks or cleaning agents like bleach. De Waal emphasised avoiding unauthorised chemical additions.
Subtle signs like gradual gas pressure drops, without pipe damage, can signal SCC. De Waal described a freon gas system using stainless steel 304L piping exposed to calcium carbonate and moisture. The combination of environmental exposure and physical stress led to cracking, only detectable via dye-penetrant testing. SCC requires both a corrosive medium and mechanical stress.
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
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