In low-temperature evaporation systems for electroplating rinse water, trace residues of chelating agents do not simply delay salt crystallisation or accelerate scale formation in isolation; both effects occur simultaneously, depending on the type and concentration of the chelating agent, the pH of the system, and the concentration ratio. The most common outcome observed in practice is initial inhibition of crystallisation followed by the induction of soft scale formation in the later stages, a mechanism entirely distinct from that of hard scale growth caused by ordinary inorganic salts.

Complexing agents such as EDTA, citric acid and tartaric acid can form stable, soluble complexes with heavy metal ions—such as nickel, copper and zinc—in water. Once the ions are complexed and encapsulated, they are unable to collide and aggregate freely to form crystal nuclei. During the early stages of concentration, when salinity has not yet reached extremely high levels, the nucleation barrier is raised; the crystallisation of metal salts that would normally precipitate easily is inhibited, and crystal precipitation is significantly delayed—this is the observed phenomenon of delayed salt crystallisation. Rinse water of the same concentration but without chelating agents will precipitate crystals in large quantities once concentrated to a certain multiple; in contrast, a feed solution containing trace amounts of chelating agents can be further concentrated, with a solid phase only appearing at a much higher salinity. At this stage, it is easy to mistakenly assume that the system is not prone to scaling, leading to a further increase in the concentration multiple and thereby creating a hidden risk of subsequent wall scaling.

As concentration continues and ionic activity rises, once the limit of the complex’s carrying capacity—corresponding to its stability constant—is exceeded, the complexation equilibrium is disrupted, and heavy metal ions are gradually released from the complexes. This decomplexation does not occur as a single, concentrated burst, but rather takes place preferentially on the heat exchange surfaces. A boundary layer exists on the heat exchange surfaces where the wall temperature is higher than that of the main liquid phase; as the temperature rises, the stability of the complexes decreases, leading to the preferential dissociation of free metal ions at the wall surface, where fine precipitates form in situ. Such deposits are not dense, hard inorganic salt crystals, but rather flocculent, gel-like hydroxides or basic salts. Adhering to the heat transfer surface, they do not possess particularly strong adhesion but exhibit excellent ductility, forming a soft scale substrate.

This soft base layer alters the surface properties of the wall, serving as an attachment site for subsequent inorganic salt crystallisation. Sulphate and chloride crystals, which are normally suspended in the liquid phase, find it difficult to adhere directly to smooth metal surfaces; however, they can easily anchor themselves to this gel-like complex scale, gradually accumulating and growing to ultimately form a composite scale layer consisting of soft scale interspersed with hard crystals. This type of scale differs from ordinary salt scale and is more difficult to remove; it is hard to dislodge by simply rinsing with water, and acid washing requires a longer contact time. Furthermore, some organic complexing components may leach out during the acid washing process and return to the original liquid tank, thereby recreating the issue of increased foam stability discussed earlier and exacerbating carryover during evaporation.

There is another side effect that is easily overlooked on site: trace amounts of complexing agents, being organic compounds, alter the surface energy at the solid–liquid interface. Even after crystals have formed, these agents adsorb onto specific crystal faces, altering the crystal growth morphology. As a result, crystals no longer grow into large, easily settleable particles, but instead form fine, needle-like or flake-like microcrystals. Such microcrystals remain suspended in the feed solution and do not readily settle to the bottom; they continue to circulate with the feed solution, coming into contact with the heat exchange surfaces, thereby increasing the likelihood of fouling and scaling. At the same time, these fine microcrystals enhance the feed solution’s tendency to foam; when combined with negative pressure conditions, the risk of mist entrainment rises accordingly.

The pH of the system is the key variable determining the intensity of these two effects. Under low-temperature evaporation conditions ranging from neutral to slightly alkaline, the stability of the complexes is high, and the effect of delaying crystallisation predominates; however, when the pH continues to rise, or in the event of localised overheating on the wall surfaces, the stability of the complexes decreases, the decomplexation reaction accelerates, and scaling rapidly becomes apparent. There is also a threshold for complexing agent concentration; at extremely low concentrations, the primary effect is the inhibition of nucleation, but once concentration and accumulation exceed a critical value, the risk of scaling rises rapidly. Repeatedly returning waste liquid to the tank after cleaning causes the complexing agent to accumulate continuously, and scaling problems that were originally inconspicuous will gradually worsen, characterised by a delayed onset.

Distinguishing between these phenomena on site also has clear characteristics: scaling caused solely by inorganic salts results in a hard scale layer with coarse crystalline particles, which precipitates once the solution is concentrated to a certain multiple; in contrast, complex scaling induced by chelating agents results in a softer scale; upon scraping, a gel-like matrix is visible, and the scale layer tends to concentrate in areas of the heat transfer surface where the temperature boundary layer is strongest. Conversely, the main body of the process fluid within the vessel contains relatively few solid particles; whilst the original liquid appears clear, scaling continues to accumulate on the vessel walls.