History of Surface Pre-Painting Using Phosphate Coatings

برگرفته از: National Metallurgical Laboratory

Translator: Parisa Atashi

خلاصه

Phosphating is one of the common pretreatment processes used for the surface preparation of ferrous and non-ferrous metals for subsequent operations such as coating and finishing. Due to its cost-effectiveness, high processing speed, and ability to provide excellent resistance to corrosion and wear, as well as improved adhesion and lubrication properties, this process plays an important role in the automotive, process, and appliance industries. Although the process was initially developed as a simple method for corrosion prevention, changes in the end-use applications of phosphated products have made it necessary to modify existing processes and develop innovative methods to replace traditional approaches. To keep pace with the rapidly changing requirements of finishing systems, numerous modifications have been made in the development of these processes, both in terms of process sequence and phosphating formulations. This review discusses various aspects of phosphating in detail. Despite numerous modifications in deposition technologies to achieve different types of coatings and desirable properties, such as improved corrosion resistance and wear resistance, phosphate conversion coatings continue to play a vital role in the automotive, process, and appliance industries.

1. مقدمه

Metals have been the backbone of human civilization. Considerable efforts have been made to find substitutes for metals; however, these materials continue to play a primary role in manufacturing and construction and are likely to retain this role for many years to come. This is due to the combination of several beneficial properties possessed by metals, such as strength, formability, low cost, and recyclability. However, metals extracted from ores through chemical or electrochemical methods exhibit a strong tendency to revert to their oxide form at the earliest opportunity; in other words, they tend to corrode [1–4]. As a result, in addition to causing enormous economic losses, corrosion poses a serious threat to a country’s national resources.

Methods for corrosion prevention are numerous and diverse. These methods can generally be classified as follows [3]:

• Modification of the Metal through Alloying and/or Surface Modification
• Modification of the Environment Using Inhibitors
• Changing the Metal/Environment Potential through Cathodic or Anodic Protection

The most common method of corrosion protection involves bulk alloying or surface modification. However, surface modification is considerably more economical than bulk alloying and is therefore more widely used. Commonly used surface modification methods involve forming a physical barrier to protect the metal from the corrosive environment [5]. This can be achieved through relatively modern methods such as: (i) physical vapor deposition (PVD); (ii) chemical vapor deposition (CVD); (iii) ion implantation; (iv) laser processing; (v) thermal spray, plasma spray, and electric arc spray deposition; (vi) nitriding; (vii) carburization; and so on, or through more traditional techniques such as: (i) painting; (ii) anodizing; and (iii) chemical conversion coatings.

While the former methods are generally less economical because they involve the use of complex application techniques and are intended for specialized applications, the latter methods are more cost-effective and have a wider range of end-use applications.

2. پوشش‌های تبدیل شیمیایی

Chemical conversion coatings are adherent, insoluble, inorganic, crystalline or amorphous surface films that form as an integral part of the metal surface through a non-electrolytic chemical reaction between the metal surface and the immersion solution [6]. In such coatings, a portion of the base metal is converted into one of the components of the resulting protective film, which is considerably less reactive toward subsequent corrosion than the original metal surface. This film imparts a uniform potential to the metal surface and neutralizes the potential of local anodic and cathodic galvanic corrosion sites [7].

They also serve as absorbent substrates to improve adhesion to paints and other organic coatings. Chemical conversion coatings are preferred due to their adherent nature and high coating formation rate, in addition to their cost-effectiveness. Furthermore, they can be formed using simple equipment without applying any external potential. Chemical conversion coating processes are classified according to their principal components into phosphating, chromating, and oxalating, which respectively involve phosphates, chromates, and oxalates [8]. This review focuses on phosphate conversion coatings, with particular emphasis on zinc phosphate coatings on mild steel.

3. فسفاته کردن

The phosphating process can be defined as a metal surface treatment process for producing a relatively hard, electrically non-conductive, and insoluble phosphate surface coating that is continuous with and highly adherent to the underlying metal, and has greater absorptivity than the metal [9]. This coating is formed as a result of a topochemical reaction that causes the base metal surface itself to become incorporated as part of the corrosion-resistant film.

3.1. تاریخچه و توسعه فرآیند فسفاته کردن

The use of phosphate coatings for the protection of steel surfaces has been known since the beginning of the twentieth century, and during this period, a major portion of the world’s production of automobiles, refrigerators, and furniture was coated using this method. The first documented evidence of the use of phosphate coatings to prevent rusting of iron and steel is a British patent granted to Ross in 1869 [10]. In the method used by him, heated iron components were immersed in phosphoric acid to prevent rusting. Since then, numerous developments have taken place, the most important of which are listed in Table 1.

Over the past 30 years, efforts have focused mainly on improving quality, particularly to keep pace with the changing requirements of modern organic coating systems. Some of the most important developments include: (i) the use of low-temperature phosphating baths to overcome the energy crisis [30–32]; (ii) the use of low-zinc technology [18,19]; (iii) the use of special additives in the phosphating bath [33–42]; (iv) the use of more than one heavy-metal ion in the existing formulation, particularly tricationic phosphating [43]; and so on. New types of phosphate coatings, such as tin, nickel, and lead phosphate coatings, have been introduced [44,45], along with the development of formulations for the simultaneous phosphating of multiple metal substrates [46,47]. The use of alternatives to conventional chromium(VI) final rinses [48] is increasing in order to comply with regulations imposed by pollution-control authorities concerning the use of chromium(VI) compounds.

The special additives used in phosphating baths are presented in Table 2, and the alternatives to chromium(VI) final rinses are presented in Table 3.

 

3.2. شیمی فسفاته کردن

All conventional phosphating solutions are dilute phosphoric acid solutions containing one or more alkali metal/heavy metal ions and essentially consisting of free phosphoric acid and the primary phosphates of the metal ions present in the bath [18,19,24,127,128]. When a steel panel is introduced into the phosphating solution, a topochemical reaction occurs in which the dissolution of iron at the microanodes present on the substrate is initiated by the free phosphoric acid in the bath. At the microcathodes, hydrogen is released.

Fe + 2H₃PO₄ ® Fe(H₂PO₄)₂ + H₂­ . (1)

The formation of soluble primary iron phosphate results in a simultaneous decrease in the concentration of free acid in the solution, which consequently leads to an increase in the pH at the solution/metal interface. This change in pH shifts the hydrolysis equilibrium between the soluble primary phosphates and the insoluble tertiary phosphates of the heavy-metal ions present in the phosphating solution, resulting in the rapid conversion and precipitation of insoluble tertiary heavy-metal phosphates [18,19,24,127,128]. In a zinc phosphating bath, these equilibria may be represented as follows:

Zn(H₂PO₄)₂ ↔ ZnHPO4 + H3PO4 , (2)

3ZnHPO₄ ↔ Zn₃(PO₄)₂ + H₃PO₄ . (3)

A certain amount of free phosphoric acid must be present to suppress hydrolysis and maintain the bath in a stable condition for effective phosphate precipitation at the microcathodes. Another factor that affects the shift in equilibrium between primary and tertiary phosphates is the bath temperature. A higher temperature facilitates easier precipitation of tertiary phosphates in a shorter period of time. Therefore, a greater amount of phosphoric acid is required for baths operating at higher temperatures. In contrast, in the case of phosphating baths operating at room temperature, there is a greater likelihood of an increase in acidity during continuous operation [129,130], which is usually neutralized by adding the carbonate of the metal forming the coating (Zn(CO₃)₂ in a zinc phosphating bath). Therefore, depending on the operating temperature and the concentration of the components present in the bath, the amount of free phosphoric acid should be selected to maintain equilibrium conditions. An excessive amount of phosphoric acid not only delays coating formation but also results in excessive metal loss.

3.3. شتاب‌دهی فرآیند فسفاته کردن

In practice, the phosphating reaction tends to proceed slowly due to polarization caused by the hydrogen evolved in the cathodic reaction. To achieve coating formation within a practical period of time, some form of acceleration must be employed. The importance of accelerating the phosphating process was recognized in the nineteenth century, and its development gained considerable momentum with the advent of the Bonderite process in 1929. More recently, Sankara Narayanan et al. [131] have reviewed the acceleration of the phosphating process and justified its role in meeting the growing demand for reduced processing time. The various methods for accelerating the formation of phosphate coatings can generally be classified into three categories: (i) chemical acceleration; (ii) mechanical acceleration; and (iii) electrochemical acceleration.

 

3.3.1. Chemical Acceleration

Oxidizing agents [132,133] and metals nobler than iron, such as copper and nickel [19], etc., constitute the most important class of chemical accelerators. They accelerate the deposition process through different mechanisms. Oxidizing agents depolarize the cathodic half-cell reaction by preventing the accumulation of hydrogen at cathodic sites, whereas noble-metal ions, through their deposition, provide cathodic sites with low overpotential and consequently promote metal dissolution [134]. Since acceleration through depolarization is preferred over merely promoting metal dissolution, oxidizing agents have found wider application than metals. In addition, they prevent excessive accumulation of iron in the bath, which can be detrimental to the formation of a good coating [25]. The most commonly used oxidizing accelerators are nitrites, chlorates, nitrates, peroxides, and organic nitro compounds, used either individually or in various combinations. Common formulations include nitrite–nitrate, nitrite–chlorate–nitrate, and chlorate–nitrobenzene sulfonic acid. The characteristics of some commonly used oxidizing accelerators are presented in Table 4.

Some reducing agents, such as alkali-metal sulfites [135], hypophosphites [136], phosphites [137], formaldehyde, benzaldehyde, hydroxylamine [138], acetaldehyde oxime [139], pyrimidine N-oxime [140], morpholine N-oxime [140], quinones [141], etc., have also been tested as accelerators, but they have not been as successful industrially as oxidizing accelerators.

 

3.3.2. Mechanical Acceleration

When a phosphating solution is sprayed under pressure onto a metal surface, coatings form more rapidly than during immersion in the same solution, because the former process eliminates the delay caused by the diffusion of solution components to the metal surface. The comparative kinetics of spray and immersion phosphating were determined by Laukonis [142]. The resulting coatings are thin, fine-grained, and highly suitable as paint bases. Other methods of physical acceleration include the action of brushes and rollers [143] on the surface during the process.

 

3.3.3. Electrochemical Acceleration

Various electrochemical methods for acceleration, including anodic, cathodic, and pulsed methods, have been described in the literature [144–161]. In 1909, Kaslet [144] identified the acceleration of the phosphating process using the cathodic method. Subsequent studies have shown that anodic methods are more suitable and beneficial than cathodic methods because they promote both metal dissolution and passivation. Zantout and Gabe [148] claimed that, by applying a small current, higher coating weights with low porosity could be achieved in a shorter treatment time. Ravichandran et al. [157–159] established the application of galvanic coupling of a steel substrate with metals nobler than steel for accelerating phosphating processes. This method uses the principle of galvanic corrosion to accelerate the metal-dissolution reaction, resulting in faster consumption of free phosphoric acid and earlier attainment of the precipitation onset point, thereby producing a greater amount of phosphate coating. The application of cathodic current for the formation of a phosphate coating on stainless steel using a calcium-modified zinc phosphating bath was patented by Bjerrum et al. [160]. Sinha and Feser [161] also studied the formation of phosphate coatings on steel and stainless-steel substrates using this method.

The three acceleration methods described above are widely used in industry; however, each has its own advantages and disadvantages. Although chemical accelerators, particularly oxidizing accelerators, accelerate the phosphating process simply by being added to the bath, their concentration in the bath is critical for achieving the desired results. Acceleration by mechanical methods is limited to spray processes, which are capable of continuously supplying fresh bath solution. Although electrochemical acceleration methods are capable of achieving higher deposition rates, the practical difficulty of introducing “electricity” into the processing stage makes them less popular.

3.4. سینتیک فرآیند فسفاته کردن

The kinetics of the phosphating process indicate the stages involved during phosphating and their rates. To date, three different methods have been used to investigate the kinetics of phosphate coating formation: (i) the gravimetric method, involving quantitative determination of the amount of phosphate deposited per unit time; (ii) the electrochemical method, based on determining the uncoated and reactive areas through electrochemical passivation; and (iii) the radiographic method, based on determining the intensity of the characteristic X-ray radiation of the resulting compound.

All three methods provided a similar picture of the phosphating process, showing that coating formation does not proceed linearly; rather, it is initially very rapid, after which the rate gradually decreases with time.

Studies on the kinetics of phosphating indicate that there are four distinct stages in coating formation (Figure 1), namely the induction stage (a), the onset of film growth (b), the main exponential growth stage (g), and the stage of linear increase in film growth (d). During the induction period, the residual oxide film on the surface is removed, even after cleaning. Once film growth begins, the first nuclei are formed, and the nucleation rate rapidly increases with time. However, this depends significantly on the surface condition, the pretreatment methods adopted, and the oxidizing agents present in the phosphating bath. Growth occurs during the main exponential growth stage. The addition of accelerators reduces the induction period and extends the linear growth stage.

According to Gebhardt [162], the phosphating rate depends on the rate of diffusion of Fe²⁺ ions from the structural lattice to the coating/solution interface through the formed coating. Machu [25] found that the rate of the phosphating reaction is a function of the microanodes present on the surface.

-dFA/dt = K.FA dt

where dt is the change in time; FA is the anodic area in the microcells; and K is the reaction rate constant.

The rate of phosphate coating formation depends primarily on the metal, i.e., on the ratio of the initially existing anodic area, FAo, to the anodic area at any given moment, FA. The effects of various other factors that control the reaction rate, such as temperature, surface condition, and so forth, are reflected in the reaction rate constant K, which differs for different processes.

The entire coating growth process can be monitored using potential–time curves (Figure 2), which illustrate the different stages of coating formation and, in addition, indicate when effective phosphating has ceased.

The correlation between the potential–time relationships and the properties of the film leads to the conclusion that coating formation proceeds through the following stages:

(a) Electrochemical attack on the steel; (b) amorphous deposition; (c) dissolution of the base metal; (d) crystallization and growth; and (e) crystal rearrangement.

In practice, however, it is difficult to identify stages “b” and “c,” and the curve mainly represents the dissolution of the metal, coating formation, and completion of the coating. The use of potential–time measurements to monitor the phosphating process was first described by Machu [25]. They have also been used by several researchers to elucidate the nature of the process during phosphating [145] and the properties of the coating formed [163]. Sankara Narayanan et al. [164,165] have discussed the usefulness of potential–time curves in predicting the kinetics of the phosphating process. The application of potential–time measurements for effective online monitoring of the phosphating process has also been demonstrated [166,167]. A correlation between coating weight and potential–time measurements was established by Sankara Narayanan [168–170], enabling the calculation of crystallization kinetics from potential–time measurements.

 

3.5. جزئیات فرآیند

In general, the phosphating sequence consists of seven operations, as shown in the flow diagram. However, depending on the surface condition of the base metal, some of these operations may be omitted, or additional operations may be added to the system. A typical seven-stage sequence of the phosphate pretreatment process is shown in Figure 3.

 

3.5.1. Cleaning

Perhaps the most important requirement for the proper formation of a coating is a clean substrate free from contaminants such as oil, grease, wax, corrosion products, and other impurities. Many coating failures can be attributed to inadequate preparation of the metal surface [171]. An ideal cleaner is one that is capable of removing all contaminants from the metal surface while preventing their redeposition or the formation of other harmful reaction products [172]. Various methods, such as sandblasting, solvent degreasing, vapor degreasing, alkaline cleaning, and pickling, have been used to achieve this objective.

Sandblasting is an effective method of mechanical cleaning. However, it is very expensive, and its use is justified as a field method where chemical methods cannot be employed and it is necessary to remove loose scale as well as paint [173].

Organic solvents are widely used to remove organic contaminants from metal substrates. However, they are toxic and flammable and require use in large quantities, making them economically impractical. This has led to their replacement by vapor degreasing. The unique advantage of the latter technique over solvent degreasing is that continuous cleaning can be achieved using small quantities of solvent [174].

Alkaline cleaning is an economical and effective alternative to the use of organic solvents for removing grease, oil, and wax. They are also used in conjunction with surfactants (wetting agents) and emulsified hydrocarbon solvents [174]. Alkaline cleaners are particularly effective when used hot (approximately 79 °C). Although alkaline cleaning is free from the fire and toxicity hazards associated with organic solvent cleaning (unless emulsified solvents are incorporated), protection against the corrosive effects of alkaline materials on the skin and ordinary clothing is necessary. Caustic soda, in particular, can cause severe burns to the skin and eyes and is highly irritating to the nasal and bronchial membranes if inhaled.

Acid cleaning, or pickling, using acids such as HCl, H₂SO₄, and H₃PO₄ is a highly effective method for removing rust and mill scale [175]. Dilute solutions (5–10 wt%) of H₂SO₄ and HCl, in the presence of inhibitors, are used to remove inorganic contaminants by converting them into iron salts. Pickling in H₂SO₄ is usually carried out at elevated temperatures (approximately 60 °C). H₃PO₄ is an excellent and time-tested cleaning agent that not only removes organic and inorganic contaminants from the metal surface but also chemically etches the surface by reacting with it to produce a mechanically and chemically receptive surface for subsequent coating formation [176].

Electrolytic pickling is an alternative to chemical pickling that provides faster and more effective cleaning through enhanced hydrogen evolution, resulting in increased turbulence and an explosive action [174].