Crystals deposited at the bottom of the chamber under low-speed circulation conditions do not cause the intense, continuous scouring seen in high-speed material flows; instead, they give rise to a form of localised erosion with a completely different wear mechanism, which is often mistaken for conventional high-speed erosion in many field applications. As deposited crystals accumulate at the bottom, the low-velocity circulating fluid flows over the surface of the accumulation layer; fine crystals in the upper layer are entrained by the fluid and slide and roll along the heat exchange plate surface. This micro-friction between the particles gradually wears away the passivation film on the heat exchange surface. Once the metal substrate loses its protection, the corrosion reaction rate accelerates significantly. This constitutes an abrasion-corrosion coupling effect, whereby scouring wear and electrochemical corrosion mutually reinforce one another.

When the circulation velocity falls precisely within the critical range, the fluid is unable to fully suspend and carry away the large crystalline particles at the bottom. As particles continue to tumble on the surface of the accumulation layer, the area of the heat exchange surface near the bottom is repeatedly subjected to particle abrasion. The passivation film is scraped away by fine crystals as soon as it forms; fresh metal is continuously exposed to the salt-containing feed solution, whilst the corrosion products are carried away by the fluid, preventing the formation of a stable protective layer. Consequently, the corrosion rate is far higher than that of simple static immersion corrosion. If defects such as weld seams or surface scratches are present on the heat transfer surface, crystals will accumulate preferentially at these locations, further amplifying localised abrasion and resulting in punctate or band-like thinning.

There is another type of operational variation that is easily overlooked: during low-speed operation, the crystals at the bottom are not stationary. Fluctuations in system vacuum, slight disturbances in feed flow rate and pump pulsations can all cause brief turbulence in the process fluid, causing crystals that were originally settled and accumulated to slide and intermittently impact the heat exchange walls. Although the energy of each individual impact is small, over many years of operation, the resulting wall thinning tends to be concentrated in the lower heat exchange region of the chamber, which differs markedly from the uniform, large-area erosion patterns seen in high-speed circulation.

Crystal accumulation also leads to an increase in heat transfer resistance and a shift in wall temperature; localised temperature rises accelerate electrochemical corrosion reactions, whilst temperature fluctuations also alter the solubility of salts, causing new crystals to continuously precipitate on the heat transfer surface. The stress generated during the adhesion and growth of these crystals, combined with particle abrasion, further damages the base material. If the feed solution contains corrosive ions such as chloride ions, the risk of pitting corrosion increases significantly once the passivation film is compromised.

Under long-term low-velocity circulation, damage caused by crystals at the bottom exhibits another characteristic: once the system switches to high-velocity cleaning mode, the large amount of accumulated crystals is instantly swept up, resulting in severe erosion within a short period. The amount of wear incurred in this brief period can even exceed the total amount of low-velocity micro-erosion accumulated over several months; this is also a contributing factor to sudden leaks in the bottom heat exchange sections of many evaporators during shutdown cleaning or when increasing flow velocity to discharge sludge.

To mitigate this type of damage, priority should be given to controlling the lower limit of the circulation flow rate to ensure that the feed stream keeps fine crystals in suspension, thereby preventing the formation of a stable accumulation layer at the bottom. Concurrently, crystal particle size should be controlled to minimise the formation of coarse, hard crystals. With regard to the selection of heat exchange surface materials, simply increasing the corrosion resistance grade is insufficient to withstand erosion-abrasion coupled damage; the material’s resistance to micro-friction wear must also be taken into account. Wall thickness in the bottom heat exchange section should be monitored regularly, and the unit should not be operated continuously for extended periods within the critical low-velocity range.