When comparing the maintenance of negative pressure via low-power heat preservation during the night with the method of breaking the vacuum and allowing the system to stand, the mechanisms of damage to the heat exchange plates are entirely different; it is not possible to simply conclude that one method is definitively better than the other. The key factor depends on the system medium—in this case, the electroplating rinse water containing trace amounts of chelating agents and heavy metal salts. From the perspective of long-term protection of the heat exchange plates, allowing the system to stand after breaking the vacuum is often the more prudent approach, though it does have its corresponding drawbacks; conversely, the continuous heating and negative pressure maintenance mode carries a risk of cumulative, latent corrosion.

When continuous heating is used to maintain negative pressure overnight, the process liquid remains within the evaporation temperature range; the gas–liquid interface persists within the chamber, and the heat exchange plate surfaces remain constantly immersed in the saline process liquid. Mass transfer within the boundary layer on the heat exchange plate walls does not cease, and heavy metal ions and chloride ions continue to react with the passivation film on the metal surface. Even in the absence of evaporation, as the wall temperature remains slightly higher than that of the main liquid phase, local variations in solubility cause fine salts and complexes to slowly precipitate onto the plate surfaces, forming an extremely thin layer of deposits. This thin scale layer creates an oxygen concentration cell; the area beneath the scale suffers from insufficient oxygen supply, forming a closed corrosion environment that gradually induces pitting corrosion. Furthermore, the presence of complexing agents continues to degrade the metal passivation film, further accelerating the corrosion rate beneath the scale. At the same time, under negative pressure conditions, the vapour space remains continuously saturated with water vapour. At the gas–liquid interface on the upper part of the plate, a continuous wet–dry cycling effect occurs, whereby volatile trace components carried by the water vapour constantly condense and evaporate on the plate surface, leading to the continuous concentration and accumulation of salts. This makes the gas–liquid interface particularly susceptible to band-shaped corrosion. Furthermore, prolonged maintenance of heat and negative pressure, combined with stagnant feed liquid or low-flow-rate circulation, causes solid particles to settle and accumulate in the flow channels at the base of the plates; as these particles adhere closely to the plate surface, they similarly form localised corrosion cells. This corrosion process progresses slowly; no changes are apparent during short-term shutdowns. However, over many years of night-time heat-retention operation, the plate wall thickness gradually thins, and in many installations, perforations in the plates are only discovered after several years of operation.

In the ‘vacuum release and standstill’ mode, once the vacuum is released, the chamber returns to atmospheric pressure; heating and temperature maintenance can then be ceased, allowing the process fluid temperature to gradually return to ambient temperature. The drop in temperature directly reduces the rate of electrochemical corrosion, as the corrosion reaction itself slows significantly with decreasing temperature. If the process fluid is simultaneously drained from the heat exchange chamber or the liquid level is lowered, the plates are no longer in contact with the fluid; the liquid-phase corrosive medium no longer comes into contact with the base material, thereby eliminating the conditions for under-deposit corrosion and concentration corrosion at the gas–liquid interface. However, there are significant risks associated with breaking the vacuum and leaving the system to stand. If the process liquid has not been completely drained, the solubility of various salts decreases upon cooling to atmospheric pressure, making it easier for crystals to precipitate and adhere to the plate surfaces. Subsequently, during restart and heating for evaporation, the interaction between these adhered crystals and the process liquid can cause abrasion. At the same time, after the vacuum is released, outside air enters the cavity, causing a significant increase in oxygen content. If a thin liquid film remains on the plate surfaces, the abundant oxygen will accelerate anodic reactions in the metal, leading to oxygen concentration corrosion in the areas where the residual film is present. This risk only arises when a residual wet film is present on the plate surfaces; it can be virtually eliminated after the unit has been thoroughly drained and blown dry. Furthermore, the repeated cycles of breaking and re-establishing the vacuum—with alternating pressure changes—subject the plate pack to slight alternating stresses. The gaskets in the plate heat exchanger are also subjected to repeated deformation. Prolonged and frequent switching between vacuum and atmospheric pressure increases the likelihood of gasket ageing and leakage. Whilst this does not constitute corrosion of the plates themselves, it represents wear and tear of associated components.

In the specific operating conditions of electroplating rinse water—where the process fluid contains heavy metal ions, trace amounts of complexing agents and salts—the greatest risk associated with maintaining a negative pressure and keeping the unit heated overnight is the continuous accumulation of complexing agents and salts in the boundary layer of the plates, leading to under-scale pitting. This is particularly damaging to the plates of a plate-and-frame heat exchanger. If the vacuum is released and the unit is left to stand, whilst simultaneously returning the process fluid from the evaporation chamber to the feed tank and performing a simple drain of the chamber to reduce the residual liquid film on the plate surfaces, this will provide better protection for the plates than maintaining a constant temperature and negative pressure. However, if on-site operating conditions are limited and it is not possible to drain the chamber, leaving the process liquid inside overnight, maintaining a low-flow-rate circulation combined with thermal insulation and negative pressure is actually preferable to static immersion at atmospheric pressure; under static atmospheric pressure, solid particles settle and accumulate, causing localised under-scale corrosion to become more concentrated.

It is also important to distinguish between two phenomena that are easily confused in the field: corrosion under continuous negative pressure conditions tends to be less uniform and is predominantly localised pitting, with perforations concentrated in the gas–liquid interface areas of the heat exchange plates and at the bottom of the flow channels; damage resulting from breaking the vacuum and leaving the material to stand is primarily due to crystal adhesion, with particulate abrasion occurring during the start-up phase, manifesting mainly as scratches and flake-like thinning.

Regarding the associated control measures: if a night-time vacuum release scheme is chosen, avoid releasing the vacuum directly whilst the system is at high temperature; instead, allow the system to cool down before releasing the vacuum to prevent thermal shock and rapid water vapour condensation. If the process necessitates maintaining a negative pressure for overnight thermal insulation, a minimum circulation flow rate must be maintained to prevent particles from settling and adhering to the plates; simultaneously, the insulation temperature should be kept as low as possible to reduce the accumulation of salts in the boundary layer.