Over nearly four decades of reform and opening-up, the metal surface‑treatment industry has experienced rapid growth, with aluminum surface treatment emerging as one of the fastest‑developing segments. This is attributable not only to the large volume of publications and patents in this field but, more importantly, to the wide variety of aluminum surface‑treatment processes. In particular, the porous nature of anodic oxide films on aluminum has enabled the development and application of numerous functional oxide coatings. Meanwhile, the continuous emergence of environmentally friendly process technologies has further steered aluminum anodizing toward greater energy efficiency, reduced emissions, and enhanced environmental sustainability.

I. Overview

When discussing clean production in the electroplating industry, material balance and the utilization and recovery rates of the plated metal serve as key indicators for assessing its effectiveness; anodizing and chemical conversion coatings on aluminum account for only a small share. Nevertheless, with the development of the national defense industry, traditional chromium‑based processes have been widely reintroduced: the fatigue‑resistant oxide film—chromic acid anodizing; the “three‑proof” oxide film—sulfuric acid anodizing combined with ********* salt filling; and the conductive chemical conversion coating known as Alodine are typical examples. The enactment of new environmental protection laws and the growing national emphasis on environmental conservation have brought about profound changes in conventional surface‑treatment processes:

First, the development of environmentally friendly process technologies has largely succeeded in replacing conventional processes that rely on hexavalent chromium. At present, these technologies have demonstrated technical feasibility at the production‑process level; however, their broader adoption will depend on whether defense‑industry design organizations adopt an open-minded stance toward these new, eco‑friendly processes.

Secondly, tank‑side separation and reuse technologies—represented by membrane processes such as RO, DD, microfiltration, and nanofiltration—are advancing toward the continuous recirculation of various process streams, with the goal of achieving substantial emissions reductions.

Finally, technologies such as seepage‑proofing for production‑line trough floors and metal‑recovery techniques in wastewater diversion and classification treatment have been implemented. The application and widespread adoption of these innovations have fundamentally transformed conventional end‑of‑pipe pollution control, shifting emission reduction from source‑level management through green processes—starting with recovery tanks, counter‑current rinsing, and reclaimed water reuse—to membrane‑based separation and recycling of bath solutions, and ultimately to the resource‑oriented utilization of hazardous waste. In this way, the principles of clean production are integrated into every stage of aluminum anodizing.

This paper focuses on the application of clean production principles in aluminum anodizing, examining environmentally friendly aluminum alloy processes, low‑waste and medium‑to‑high‑waste production line solutions, and transformative practices in tank‑side treatment.

II. Environmentally Friendly Process Technologies for Aluminum Alloy Surface Treatment

As public awareness of environmental protection continues to grow, national environmental policies are being continuously refined and strengthened. In addition to heavy metals, discharge standards for Category II pollutants—such as total phosphorus, total nitrogen, ammonia nitrogen, and COD—have been raised to varying degrees. Consequently, the widespread adoption of environmentally friendly processes in aluminum surface treatment has become an inevitable trend.

(1) Environmentalization of the Pre-treatment Process

In aluminum alloy anodizing production, the typical pretreatment sequence consists of three steps: degreasing (oil removal), deoxidation (alkaline etching), and neutralization (brightening). Two common degreasing processes are neutral/weakly alkaline degreasing and acidic degreasing. Among commercially available additives, formulations containing phosphorus, nitrites, or with a COD exceeding 2,000 ppm can significantly complicate wastewater treatment and environmental compliance. If a neutral or weakly alkaline degreaser is preferred, SY‑202 (aluminum degreasing powder) is a phosphate‑free, nitrogen‑free, low‑COD weakly alkaline product, with a COD below 1,000 ppm. To preserve the surface luster of aluminum during pretreatment, a room‑temperature option such as SY‑201 (acidic aluminum degreaser)—commonly referred to in the industry as “aluminum acid degreasing”—is recommended. For the pretreatment of cast aluminum alloy surfaces, where oxide scales, cold shuts, and release agents must be removed, SY‑208 (cast aluminum piston cleaner), used under ambient conditions, enables one‑step cleaning that effectively addresses all these contaminants.

The etching‑washing step in aluminum pretreatment uses a simple bath formulation: an alkaline solution containing caustic soda and aluminum‑complexing additives such as sodium gluconate or sorbitol. While environmentally friendly, this bath exhibits slow cleaning performance and excessive etching, which inevitably increases the consumption of alkali and accelerates bath degradation. By employing SY‑204 (an****** alkaline etchant), it is possible not only to achieve a high‑quality E6 (satin) finish but also to reduce the etching rate by a factor of three, thereby achieving both emission reduction and resource conservation.

For the neutralization solution used in the anodizing of wrought and cast aluminum alloys, nitric acid is not required; however, it must possess oxidizing properties.

Therefore, it is essential that the neutralizing solution possess a certain degree of oxidizing power: first, to help dissolve alkaline corrosion products, including alloy‑related black residues (such as copper and manganese) and silica ash; second, the neutralization‑and‑de‑ashing process also serves to form a “pre‑passivation film” on the surface of wrought aluminum alloys. With this pre‑formed chemical oxide layer, the aluminum alloy surface can both prevent inter‑process corrosion during electrolytic or chemical oxidation and enhance the corrosion resistance of the resulting oxide film. SY‑211 (a de‑ashing and pre‑passivation agent) is precisely such a product. After replacing nitric acid with an alternative neutralizing solution, the treated effluent can be discharged directly—this approach is highly favored in clean production.

(II) The Environmental-Friendly Trend in Oxidation-Based Film-Forming Processes

Anodizing of aluminum that involves the heavy metal hexavalent chromium primarily comprises chromic‑acid anodizing and chromate‑based anodic oxidation; the former is a standard process in the aerospace industry for producing fatigue‑resistant oxide films, while the latter is used to form insulating oxide layers in certain electronic products. Environmentally friendly, fatigue‑resistant oxide‑forming processes that can replace chromic‑acid anodizing include boron‑sulfuric‑acid anodizing, whose performance fully matches that of chromic‑acid anodizing. At present, Europe and Japan have recognized boron’s impact on ecosystems and have begun to impose restrictions on its emissions; China is also set to enact legislation limiting boron discharges. As for alternative processes to “porcelain‑like oxide films,” numerous options exist, with the potassium titanium oxalate method delivering particularly excellent results and enjoying the broadest application.

In the field of chemical conversion coatings, Alodine 1200 and 1200S are benchmark processes for chromating or phospho‑chromating aluminum alloy surfaces. However, their formulations rely on high concentrations of hexavalent chromium, which has imposed significant environmental pressures, making the development and adoption of environmentally friendly alternatives to hexavalent chromium an inevitable trend. The SY‑323 developed by Sanrong—a trivalent chromium passivator for aluminum alloys with enhanced salt‑spray resistance—has already surpassed these two Alodine products in corrosion resistance.

(3) Post‑treatment of the oxide film – sealing process

The sulfuric acid anodized film on aluminum alloys is sealed by filling and passivation using a specialized salt‑based formulation, constituting the “three‑protective” anodizing coating widely employed in marine‑grade aluminum alloy components. Through the sustained efforts of surface‑treatment researchers, numerous high‑performance, environmentally friendly sealing technologies for anodic films have been successfully developed. As early as 2013, Sanrong Company publicly disclosed a patent entitled “An Oxide Film on Aluminum or Aluminum Alloy Surfaces and Its Preparation Method” (Patent No.: ZL201310498581.1)—the so‑called “three‑protective anodizing film”—which has since been applied to a variety of shipboard service‑life products.

As the environmental friendliness of aluminum alloy surface‑treatment processes continues to advance, regulatory requirements for nickel emissions are becoming increasingly stringent. According to Table 3 of GB 21900‑2008, “Emission Standards for Electroplating Pollutants,” the permissible concentration limit for nickel has been lowered to 0.1 mg/L. Clearly, whenever a process formulation contains nickel, achieving compliance in wastewater treatment is extremely challenging; thus, nickel‑free, environmentally friendly sealing technologies have emerged as a key solution. SY‑333, a medium‑temperature, nickel‑free sealing agent that is fluorine‑free and free of heavy metals, is ideally suited for sealing both natural‑color and electrolytic coloration oxide films. This product has undergone six years of production‑scale validation, with a mature and reliable formulation and process.

For heavy-duty anti-corrosion anodized films that demand high corrosion resistance, organic sealing agents or nano‑based sealing products are sometimes employed. However, these approaches can increase the COD level in wastewater, thereby raising treatment costs, and for products subject to dyne‑force testing, the use of organic sealants may adversely affect test results. At Sanrong, we enhance the protective performance of workpieces by applying mechanical vibratory polishing. For products without dyne‑force specifications, polyethylene wax (liquid) is used as the polishing agent; for those with dyne‑force requirements, SY‑131—a mirror‑finish polishing compound for aluminum—can be employed.

3. Tank-Edge Treatment Technology for Aluminum Anodizing Production Lines

The tank‑side treatment system for aluminum anodizing production lines employs a flexible combination of membrane technologies, drying techniques, and chemical/electrochemical reactions to remove impurities from the bath solution. This enables the bath to be recycled or significantly extends its service life, thereby fundamentally reducing waste‑liquid generation. Depending on the specific characteristics of each anodizing process step, an appropriate tank‑side treatment method must be selected:

(1) Online Reuse Equipment for Degreasing Tank Solution

The key to online treatment of degreasing tank liquor is the removal of floating oil. A cleverly designed oil‑water separator can effectively integrate demulsification, oil–water separation, and sludge removal into a single unit (refer to the patent schematic). The oil‑removal equipment operates on a multi‑stage separation principle, efficiently separating floating oil and impurities from the solution; a rotating scraper equipped with an oil‑skimming blade then extracts the floating oil at the tank opening. For systems requiring demulsification, a separate dosing and aeration tank must be provided; this aeration tank serves as the primary inlet for the solution. The impurity‑settling tank features a conical bottom, allowing settled contaminants to be discharged through a low‑level outlet.

(II) A Proposal for the Recovery of Aluminum from Alkaline Etching Bath—Recrystallization Technology

In the early stages of reform and opening-up, China introduced from Japan a caustic‑solution recovery unit based on a crystallization tower. Its operating principle relies on the high‑ and low‑temperature recrystallization of alumina to recover aluminum oxide from the solution. Saturated alumina in the hot caustic solution is passed over plastic louvers coated with fine alumina powder; the alumina powder serves as a seed that accelerates the precipitation of saturated alumina crystals. At the top of the crystallization tower, a high‑capacity induced draft fan cools the stream, while continuous circulation of the liquor enables precise control of aluminum ion concentrations.

(3) Aluminum Removal Scheme in Anodizing Electrolyte—Anion Diffusion Dialysis

The electrolyte in anodizing tanks primarily consists of various acids, with sulfuric acid anodizing accounting for approximately 90% of industrial production. Over time, the dissolving action of the acid leads to a gradual accumulation of aluminum ions in the solution. When the aluminum ion concentration exceeds 15 g/L, the surface quality of the anodized products is adversely affected. To ensure product quality, it is customary to periodically monitor the aluminum ion concentration in the bath; if the concentration becomes too high, a portion of the bath should be drained and replaced with fresh electrolyte to reduce the aluminum ion level.

Diffusion dialysis is a process that “filters” aluminum ions from an aqueous solution using a diffusion dialysis membrane, as illustrated in the figure below. High-valence ions such as aluminum ions cannot pass through the membrane, whereas anions like sulfate ions can permeate freely. Under the driving force of the concentration gradient, large amounts of sulfate ions are carried away by the water on the permeate side, while the aluminum ions become concentrated in the retentate.

The actual case involves an anodizing bath containing 13 g/L of sulfuric acid at a concentration of 190 g/L.   Taking aluminum ions as an example, the treated dilute acid has a concentration of at least 140 g/L, with aluminum ion concentrations below the detection limit (less than 0.5 g/L), whereas the concentrated aluminum‑containing waste liquid has a concentration of 26 g/L. Subsequently, by adding a small amount of concentrated sulfuric acid to the dilute acid, a fresh plating bath can be prepared; meanwhile, the aluminum‑containing waste liquid, which contains only trace amounts of other impurities, can be concentrated and dried to yield aluminum sulfate, a water‑purification agent.

The benefit calculation for this process is as follows:

Tray capacity: 12–13 tons

After one month of use, the new plating bath can achieve an aluminum concentration of up to 20 g/L.

Aluminum should be maintained below 10 g/L (ideal condition: aluminum ion concentration of 4–5 g/L).

Aluminum dissolution efficiency calculation:

Assuming the aluminum concentration in the槽 (tank) is 4 g/L, and with 20 working days per month, the daily increase in aluminum concentration corresponds to an approximate dissolution of 10 kg of aluminum per day.

If the aluminum concentration in the bath is maintained at 4 g/L, the daily volume of bath solution to be treated would be 2.5 tons per day.

(4) Phosphoric Acid Recycling Technology for Electrolytic Polishing Tanks

Electrochemical polishing solutions are typically composed of a specific ratio of sulfuric acid and phosphoric acid. During operation, if the concentration of dissolved aluminum becomes excessively high, it can impair the dissolution rate on the workpiece surface during polishing. Moreover, due to the highly corrosive nature of the polishing solution, conventional membrane materials struggle to maintain long-term resistance to corrosion.

To address the aforementioned issues, a two‑step approach is proposed: first, dilute the solution to bring its concentration within a range safe for the membrane system; then, employ either nanofiltration or a combination of diffusion dialysis and reverse osmosis to remove aluminum ions from the solution. Nanofiltration is a cutting‑edge molecular‑based membrane separation technology and currently one of the most actively researched areas in the field of membrane separation. Compared with reverse osmosis, it operates at lower pressures and is therefore also referred to as low‑pressure reverse osmosis. Initially, the polishing bath effluent is diverted and combined with the concentrated streams from the recovery and rinsing tanks after passing through a nanofiltration membrane, thereby reducing the solution’s concentration to a level that ensures membrane safety. The nanofiltration membrane then selectively retains aluminum ions, while the permeate stream is recovered. The acid‑rich permeate can be reused after acid supplementation or further purification. Meanwhile, the concentrated aluminum‑bearing solution generated during the process can be treated using methods analogous to those employed in diffusion dialysis.

(5) Reuse Technology for Greywater in Conventional Washing Tanks

The equipment first separates the rinse water and employs a highly precise method to remove trace inorganic acid anions, metal ions, solid impurities, and organic compounds from the rinse‑tank effluent on the production line, enabling the rinse water to be recycled. Compared with the working tank, the conventional wash tank has a lower concentration and relatively stable component ratios; by concentrating the solution or using reverse osmosis membranes, the concentrated stream can be separated and returned directly to the working tank, while the purified water can be repurposed or routed back to the pure‑water tank.

Slot‑edge treatment is an emerging equipment sector in the metal surface‑treatment industry, focused on end‑of‑line pollution control and resource recycling. Although this technology is not yet fully mature and remains constrained by the relatively underdeveloped state of domestic membrane technologies, its advent has marked a significant step forward, moving surface‑treatment production lines from automated to intelligent operations and making zero‑emission operation for aluminum anodizing lines a practical reality.

IV. Summary

As the concept of clean production gains wider acceptance, the methods for implementing aluminum anodizing processes are continually being refined. Traditional clean‑production practices in metal surface‑treatment lines—such as tank‑end management, recovery tanks, counter‑current rinsing, and reclaimed‑water reuse—have become standard features of modern production lines. Furthermore, innovations like acid‑mist suppressants, alkali‑mist inhibitors, Venturi nozzles that optimize solution agitation and mixing efficiency, bundle‑burner units for removing impurities and converting trivalent chromium in chromic‑acid anodizing and alodine baths, cathode shields that prevent hydrogen absorption at the cathode from entraining acid mist, micro‑electrolytic decontamination technologies, and advanced wastewater‑treatment techniques—including microfiltration for concentration, vacuum evaporation for thickening, and spray‑drying—are all contributing to a substantial improvement in the level of clean production in aluminum surface treatment. With the concerted efforts of researchers and practitioners in the field, we are confident that groundbreaking clean‑production equipment and technologies for aluminum surface treatment will be developed, paving the way for environmentally friendly processing methods in this industry.

E-mail:
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Company Manager: Mr. Cui
Phone:+86 18045023377

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Phone:+86 13674601816

Address:
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