To master and manage the degreasing process properly, one must correctly understand the principle of adhesion between the coating and the metal substrate. This point is often overlooked, which creates difficulties in practice.
Relevant literature indicates that mechanical bonding caused by microscopic roughness between the coating and the substrate surface can be strong only when intermolecular and intermetallic forces are established between the coating and the metal substrate. These intermolecular and intermetallic forces can only manifest over very small distances.
When the distance between molecules exceeds 5 μm, intermolecular forces cease to act. Therefore, even a thin oil film or oxide film on the substrate surface can hinder intermolecular or metallic bonding.
To achieve the above bonding, oil, rust and scale on the workpiece must be removed quite thoroughly. By "quite thoroughly," we do not mean that the surface must be absolutely clean after pretreatment, but only that it must have a qualified surface. A so-called qualified surface in fact means that after pretreatment, any film that would hinder electroplating must be removed and replaced by a film that readily accepts electroplating.
In addition, pretreatment requires that the metal surface be absolutely smooth. Through mechanical treatments such as grinding, polishing, barrel finishing or sandblasting, obvious defects such as scratches and burrs are removed so that the substrate surface meets the leveling and finish requirements of the plated part before degreasing and rust removal.
This point must be clear. Only when it is clear can the proper pretreatment process sequence and formulation be selected correctly and realistically among similar pretreatment formulations.
How is the degreasing process applied in production?
Alkaline degreasing is commonly used. The degreasing solution composition and process conditions are selected according to the oil contamination state and the type of metal material.
When the surface is covered with a large amount of grease, i.e., a thick, slippery and sticky oil layer, alkaline degreasing alone cannot easily remove it. Other methods, such as solvent degreasing by brushing or wiping, must first be used as pretreatment, followed by alkaline degreasing. Alkaline degreasing solutions are strongly alkaline. They can react with some metals and cause noticeable corrosion.
Therefore, for workpieces such as aluminum and zinc, degreasing should be carried out under low-temperature and low-alkalinity conditions as much as possible. Iron and steel parts can generally be treated with higher alkalinity. However, when treating nonferrous metal parts, the pH of the degreasing solution should be adjusted to an appropriate range. For example, for aluminum, zinc and their alloys, pH should be kept below 11, and the degreasing time for such products should not exceed 3 min.
From a cost perspective, some advocate low-temperature degreasing, but lowering the temperature conflicts with improving efficiency. The higher the temperature, the faster the physicochemical reaction between the grease adhering to the surface and the cleaning agent, and the easier the degreasing.
Practice has shown that the viscosity of oil contamination decreases as temperature rises, so degreasing proceeds more easily, but low temperature does not have this effect. Therefore, the use of emulsifiers and surfactants should be considered. Is high-temperature degreasing good or not, and what temperature should be controlled? In the author's experience, 70–80 °C is preferable. This can also help eliminate residual stress in the base metal caused by machining, which is very beneficial for improving the adhesion of coatings, especially multi-layer nickel.
For general iron and steel parts, combined degreasing can be used, such as cathodic degreasing for 3–5 min followed by anodic degreasing for 1–2 min, or anodic degreasing for 3–5 min followed by cathodic degreasing for 1–2 min. This can be achieved by two-step degreasing or by using a power supply with a reversing device.
For high-strength steel, spring steel and thin parts, only anodic degreasing for a few minutes is used to prevent hydrogen embrittlement. However, nonferrous metal parts such as copper and copper alloys must not be anodically degreased; only cathodic degreasing for 1–2 min is allowed.
Regarding the preparation and maintenance of degreasing solutions. The preparation of chemical degreasing and electrolytic degreasing solutions is relatively simple. First, use 2/3 of the tank volume of water to dissolve all materials except the surfactant, while stirring (to prevent caking of chemicals). Since these chemicals release heat when dissolving, no heating is needed. The surfactant should be dissolved separately in hot water and then added. If it cannot be dissolved in one batch, pour off the clear supernatant, add more water to dissolve it, then make up to the specified volume and mix evenly before use.
The following points should be noted in degreasing solution management: ① Regularly analyze and replenish chemicals. The surfactant should be replenished weekly or every half month by 1/3 to 1/2 of the original amount, depending on production volume.
② The iron plates used should not contain excessive heavy metal impurities, so as to prevent them from being introduced into the coating. The current density should be kept at 5–10 A/dm2, and it should be selected so as to ensure sufficient bubble evolution. This both ensures that oil droplets are mechanically stripped from the electrode surface and stirs the solution. For a given amount of oil contamination on the surface, the higher the current density, the faster the degreasing.
③ Floating oil contamination in the tank should be removed promptly.
④ Sludge and dirt in the tank should be cleaned regularly, and the bath solution should be replaced in a timely manner.
⑤ Low-foaming surfactants should be used in the electrolyte as much as possible; otherwise, if carried into the electroplating bath, they will affect quality.
How should the acid pickling (etching) process be mastered and managed?
Like the degreasing process, acid pickling (etching) holds an important position in pretreatment. The two are used together in pretreatment production, and their main purpose is to remove rust and scale from metal parts to be plated.
Generally, pickling used to remove large amounts of oxides is called strong pickling, while pickling used to remove thin oxide films that are difficult to see with the naked eye is called weak pickling. These can be further divided into chemical pickling and electrochemical pickling. Weak pickling is used after strong pickling as the final treatment step before the workpiece enters the electroplating process; it is a surface activation process. It is easily overlooked in production, and this is precisely one of the causes of peeling of electroplated coatings.
If the weak pickling solution is one of the components of the next plating solution, or if its carry-over will not affect the plating solution, it is best not to rinse the workpiece but to put the activated parts directly into the plating bath.
For example, the dilute acid activation solution used before nickel plating. To ensure smooth pickling, degreasing must be carried out before pickling; otherwise, the acid and metal oxides cannot make good contact, and the chemical dissolution reaction will be difficult to proceed.
Therefore, to master acid pickling well, one must also understand these basic principles theoretically.
Generally, sulfuric acid and hydrochloric acid are mainly used for acid pickling to remove scale from iron and steel parts. The method is simple, but in actual production, if attention is not paid, it is difficult to achieve the desired result.
The selection standard for sulfuric acid pickling process conditions is usually based on experience, judged by the appearance of the workpiece after pickling. After all, this cannot be quantitatively controlled. Practice has shown that the descaling effect of sulfuric acid pickling at 40 °C is much greater than at 20 °C, but as the temperature increases further, the stripping effect does not increase proportionally.
Meanwhile, in sulfuric acid concentrations below 20%, the pickling rate increases with increasing concentration, but when the concentration exceeds 20%, the pickling rate actually decreases. Therefore, we consider 10%–20% sulfuric acid concentration and pickling below 60 °C to be suitable standard process conditions. Attention should also be paid to the aging degree of the sulfuric acid solution. Generally, when the iron content in the pickling solution exceeds 80 g/L and the ferrous sulfate content exceeds 2.5 g/L, the sulfuric acid solution can no longer be used.
At this point, the solution should be cooled to allow the excess ferrous sulfate to crystallize out and be removed, then fresh acid should be added until the process requirements are met.
For hydrochloric acid pickling, the selection of process conditions generally requires a concentration of 10%–20%, carried out at room temperature. Compared with sulfuric acid, under the same concentration and temperature, hydrochloric acid pickling is 1.5–2 times faster than sulfuric acid.
Whether sulfuric acid or hydrochloric acid should be used for pickling depends on the specific conditions of actual production. For example, in strong pickling of ferrous metals, sulfuric acid or hydrochloric acid is often used, or a "mixed acid" prepared by combining the two in a certain proportion.
However, the choice of acid for chemical strong pickling should be based on the composition and structure of the oxide scale on the surface of steel and iron parts. At the same time, the pickling rate should be fast, production cost should be low, and dimensional deformation and hydrogen absorption of metal products should be avoided as much as possible. It must be understood that in hydrochloric acid, scale removal mainly depends on its chemical dissolution of the scale, while the mechanical stripping effect of hydrogen gas is much smaller than in sulfuric acid. Therefore, when hydrochloric acid is used alone, acid consumption is somewhat greater than when sulfuric acid is used alone.
When the rust and scale on the surface of the parts to be plated contain a high proportion of ferric oxides, mixed acid pickling can be adopted. This not only gives play to the tearing effect of hydrogen on the scale, but also accelerates the chemical dissolution of the oxides. However, if the metal surface only carries loose rust products (mainly Fe2O3), hydrochloric acid alone can be used for pickling, because its pickling rate is fast, the dissolution of the base metal is small, and hydrogen absorption is also less.
But when the metal surface has compact scale, using hydrochloric acid alone consumes more acid and costs more, and its stripping effect on scale is inferior to that of sulfuric acid; sulfuric acid is therefore preferable.
Electrolytic pickling (electrolytic acid pickling, electrochemical pickling), whether cathodic electrolysis, anodic electrolysis, or PR electrolysis (periodic reverse electrolysis, in which the polarity of the workpiece is periodically reversed), can all be carried out in a 5%–20% sulfuric acid solution.
Compared with chemical pickling, electrolytic pickling can remove firmly adherent scale more quickly, causes less corrosion to the base metal, and is easier to operate and manage; it is suitable for automatic electroplating lines. In Japan, the PR electrolysis method is widely used to remove scale from stainless steel.
In China, anodic and cathodic electrolytic acid pickling combined with electrolytic degreasing is used by many plants for pretreatment before plating. Anodic electrolytic acid pickling of ferrous metals is suitable for treating metal parts with large amounts of scale and rust, and can mostly be carried out at room temperature. Raising the temperature can increase the pickling rate, but not as much as in chemical pickling. Increasing the current density accelerates pickling, but if it is too high, the base metal becomes passivated.
At this time, the chemical and electrochemical dissolution of the base metal essentially disappears, leaving only the stripping effect of oxygen on the scale. Therefore, the pickling rate increases very little. This characteristic must be well mastered; a current density of 5–10 A/dm2 is generally appropriate. For anodic acid pickling, o-xylyl thiourea or sulfonated woodworking glue can be used as inhibitors at a dosage of 3–5 g/L. For cathodic electrolytic acid pickling of ferrous metals, a sulfuric acid solution can be used, or a mixed acid containing about 5% sulfuric acid and 5% hydrochloric acid, with an appropriate amount of sodium chloride added. Because it has no obvious chemical and electrochemical dissolution process of the metal base (iron), the appropriate addition of Cl- containing compounds can help loosen the scale on the part surface and accelerate the pickling rate; formaldehyde or urotropine can also be used as inhibitors.
In short, sulfuric acid is widely used for acid pickling of steel, copper, and brass. In addition to the above, sulfuric acid, together with chromic acid and dichromates, is used as a deoxidizing and de-smutting agent for aluminum.
It is used together with hydrofluoric acid or nitric acid, or with both, to remove scale from stainless steel. The advantage of hydrochloric acid is that it can effectively pickle many metals at room temperature; one of its disadvantages is that attention must be paid to preventing pollution from HCl vapor and acid mist.
In addition, nitric acid and phosphoric acid are also commonly used in manual pretreatment before plating. Nitric acid is an important component of many bright pickling agents. Mixed with hydrofluoric acid, it is used for removing heat-treatment scale on aluminum, stainless steel, nickel-based and iron-based alloys, titanium, zirconium, and certain cobalt-based alloys.
Phosphoric acid is used for rust removal on steel parts, and can also be used in special bath solutions for stainless steel, aluminum, brass, and copper. A phosphoric acid–nitric acid–acetic acid mixed acid is used for pretreatment of aluminum parts before bright anodizing. Fluoroboric acid has been proved to be the most effective pickling solution for lead-based alloys or copper or brass parts with tin solder.
Reports have indicated that the removal of metal scale and oxides consumes 5% of the world’s sulfuric acid output, 25% of its hydrochloric acid output, the vast majority of hydrofluoric acid, and large amounts of nitric acid and phosphoric acid.
Therefore, correctly mastering the use of these acids for pickling is obviously an important subject in the applied technology of pretreatment before plating. But knowing how to use them is not difficult; using them well, economically, and with reduced consumption is no easy matter.
The degreasing and pickling processes described above are pretreatment processes commonly used by electroplating plants in urban and rural areas of China at present. These processes are undoubtedly effective.
Therefore, further research and improvement in application technology is of great practical significance. From a developmental point of view, however, it must be recognized that these processes still have some shortcomings needing urgent improvement: ① Chemical or electrolytic degreasing and pickling consume large amounts of electrical and thermal energy.
For China, where the contradiction between energy supply and demand is becoming increasingly prominent, energy conservation is an urgent issue to be solved in current enterprise reform.
Calculations have proved that on a medium-sized decorative nickel-chromium electroplating straight-line automatic line, the electrical energy consumed by electrochemical degreasing alone accounts for about 20% of the total electricity consumption of the entire line.
In addition, the heat energy consumed for heating and holding temperature (steam heating) in chemical and electrolytic degreasing accounts for 40%–80% of the total heat energy of the entire line;
② Many factors affect degreasing and pickling, and the applicability is especially poor for workpieces with complex geometric shapes;
③ It adds power supply equipment, heating tubes, boiler coal consumption, and exhaust equipment. Moreover, there are many pretreatment tank stations, large floor area, high energy consumption and cost, and difficult maintenance. With even slight mismanagement, alkali mist, acid mist, and the like will pollute the environment and affect quality.
In recent years, with China’s economic reform and the development of energy-saving science and electroplating technology, many metal cleaning agents that can replace gasoline cleaning have continuously emerged on the market, as well as various energy-saving degreasing and rust-removing cleaning agents and additives for corrosion inhibition and mist suppression. These new products, together with the emergence of one-step degreasing and rust-removing processes, have quickly shown their vitality in application.
Their common characteristics are low-temperature or normal-temperature operation, good static energy-saving effect, safety, and non-toxicity. At present, many domestic manufacturers are also constantly introducing and exploring advanced pretreatment processes and cleaning agents for degreasing and rust removal. Obviously, these are encouraging development trends.
How should they be applied amid continuous exploration?
In our experience, attention should be paid to mastering the pretreatment of manual production lines. Because the operating sequence is relatively flexible, choices can be made at any time according to the specific situation.
Because a manual line is more adaptable than an automatic plating line, these processes cannot be mechanically transferred to automatic lines. When a new process or cleaning agent is used on an automatic line, many issues must be repeatedly demonstrated; otherwise, overeagerness for quick success will cause large-batch losses.
However, new cleaning agents that truly save energy and reduce consumption can be adopted gradually, first as suitable replacements while retaining one electrolytic degreasing step, and then progressively improved. In addition, when trialing the above new products, because the movements of automatic lines are mechanically limited, the defoaming property and cleaning performance of the cleaning agents in application must be strictly required to prevent drag-out and contamination of the plating bath that could cause failures.
Moreover, the degreasing and derusting time should not be too long; both operation and cost must be considered.
In short, the current development of these new pre-treatment processes and cleaning agents aimed at saving energy and reducing consumption is undoubtedly the direction of electroplating reform and must be fully affirmed and upheld. It is hoped that relevant research, manufacturing, and application units will cooperate across fields, make strenuous efforts in application technology, and strive to open a new path in pre-plating treatment.
Research on the role and application technology of cleaning: After degreasing and pickling, the residual solution and residual salts adhering to the plated parts must be thoroughly rinsed, or combined with measures such as neutralization and desmutting, to continuously and repeatedly clean the metal surface, reduce surface tension, and minimize drag-out losses, so that electroplating failures during operation can be eliminated in the pre-plating treatment.
But cleaning workpieces with water seems simple, yet is by no means easy. To make cleaning work scientific and reasonable, and to coordinate closely with degreasing, pickling, and neutralization processes, so as to improve plating quality and also save water, energy, and reduce consumption, there are many application technology issues worth studying.
With the continuous development of electroplating production, how to reduce bath drag-out and enhance the effectiveness of rinsing through improved cleaning methods is an important link in ensuring the corrosion resistance and decorative performance of coatings.
At the same time, it is necessary to minimize the amount of cleaning water used while ensuring that the workpieces are cleaned thoroughly.
Different cleaning methods have different cleaning effects. For example, countercurrent rinsing uses far less water than other cleaning methods, which is beneficial to wastewater treatment and recovery and reuse of water. Therefore, countercurrent rinsing is being increasingly promoted in electroplating production. However, attention should be paid in application to: ① the characteristics of multi-stage countercurrent rinsing in application.
The earliest cleaning method used in electroplating production was single-tank rinsing. Multi-stage segmented rinsing can save a large amount of water compared with single-tank rinsing, but it increases the number of tanks, occupies more floor space, and complicates operation. Multi-stage countercurrent rinsing improves on the shortcomings of the former; it saves more than 95% of water compared with single-tank rinsing.
Generally, three tanks (stages) are used. Because three-stage countercurrent rinsing uses little water, it is convenient to realize the idea of circulation and repeated water use, with little or no wastewater discharge, so as to achieve the goal of "closed-loop circulation," which is conducive to electrolyte recovery and environmental protection.
② Multi-stage countercurrent rinsing production process.
Three-stage rinsing is usually adopted. To overcome the poor rinsing effect caused by uneven concentration in the tank due to low water consumption, oil-free compressed air or ultrasonic devices are generally used.
Interconnected tanks are used for rinsing, and the design generally adopts continuous water supply for flow rinsing. For the inlet and outlet positions of the rinsing tanks, it is advisable to feed water from the bottom and discharge from the top, which can provide a certain stirring effect.
As for the tank size, while meeting the smooth operation of the largest workpieces and racks, attention should be paid to maintaining a relatively high water surface flow rate and renewal of the cleaning water.
In addition, a baffle should be added between every two interconnected tanks to prevent water from overflowing into adjacent tanks due to rising water level during rinsing, which would affect rinsing quality.
Although there is no unified national standard in China for the allowable range of dissolved substances and impurities in cleaning water, the highly toxic cyanide ions and hexavalent chromium should be strictly controlled in accordance with relevant process wastewater standards.
Therefore, for the rinse between two process steps, care must be taken not to affect the quality of the next step. For the rinse after immersion in hydrochloric acid or sulfuric acid before chromium plating, excessive Cl⁻ and SO₄²⁻ must be prevented from being carried into the chromium plating bath.
Thus, they should be strictly controlled within low concentration limits. For the rinse after alkaline cleaning and before acid pickling, the concentration limit is relatively broad and can be between 1 and 3 g/L.
It can be seen that rinsing is an indispensable step in the electroplating process flow, and the issue to be addressed now is to attach importance to management improvement and enhancement in this regard.
In short, improving and upgrading the pre-plating treatment process and management is a strategically significant task in the current electroplating reform.
Only by continuously improving the pre-plating treatment process and the quality of scientific management can we prevent faults that could have been eliminated in pre-plating treatment from being carried into the electroplating process and causing a large number of rejects. Clearly, this is an urgent issue for ensuring electroplating quality.