Trace amounts of chelating agents exhibit a dual effect in low-temperature evaporation systems for electroplating rinse water; it cannot be simply concluded that they merely delay or accelerate scaling. Their behavior is completely opposite at the beginning and end of the entire concentration process, which is also the core reason why crystallization does not occur easily in the early stages of evaporation but is suddenly followed by extensive scaling later on.

In the early stages of concentration, the salt concentration in the original solution is relatively low, and the chelating agent in the system preferentially forms stable, soluble complexes with heavy metal ions such as nickel, copper, and zinc. With the metal ions encapsulated by the chelating agent, they cannot freely collide and combine to form crystal nuclei; consequently, even when the solution reaches its theoretical saturation concentration, crystals fail to precipitate, significantly delaying salt crystal formation. During this phase, operation is stable, and no crystallization is visible to the naked eye on the heat exchange surfaces; many operators mistakenly assume that the water quality is not prone to scaling. At the same time, the complexes exert a certain dispersing effect, so even if a small number of fine crystals form, they remain suspended in the liquid phase and do not readily adhere to the heat exchange surfaces.

As concentration continues, salt content accumulates, and ion concentration rises steadily, disrupting the equilibrium. On one hand, the concentration of free metal ions exceeds the complexing agent’s capacity limit, and excess metal ions are no longer encapsulated; on the other hand, low-temperature evaporation under negative pressure continuously removes water, causing the system temperature and ionic strength to rise. As a result, the stability of the complexes gradually decreases, and some complexes dissociate, releasing large amounts of free heavy metal ions. At this point, there is a brief, explosive formation of a large number of new microcrystals; these numerous fine grains possess an extremely high specific surface area and strong adsorption capacity.

These microcrystals no longer remain uniformly suspended and are more likely to adhere to the heat exchange plates and chamber walls. Unlike the crystallization of ordinary inorganic salts, the trace organic components and hydrolyzed hydroxides produced by the decomposition of chelating agents act as binding agents, causing the crystals to adhere firmly to the metal surface and form a dense, sticky scale layer—which is the “wall scaling” observed on-site. While ordinary salt scale can be partially removed through circulation flushing, this type of composite scale containing organic matter has extremely strong adhesion and is difficult to remove via conventional water circulation flushing; its continuous accumulation reduces heat transfer efficiency.

In addition, chelating agents exacerbate foam formation; rising foam carries fine crystals to the upper part of the chamber and the demister zone, further expanding the scope of wall scaling. The problem is further magnified during shutdown and standstill conditions. In a static environment lacking fluid shear, the complexes slowly dissociate, and microcrystals continue to deposit on the walls; a thickening of the scale layer can be observed with each shutdown and restart.