Why does direct pyrophosphate copper plating of steel parts have poor adhesion?

Yuan Shipu

(Chengdu Jitou Town Clubhouse Garden A3202202, Chengdu, Sichuan 610045)

Abstract: Copper pyrophosphate plating has not yet been a precedent for the industrial successful application of steel parts, zinc die castings and galvanized aluminum parts without pre-plating and direct plating. Under the condition of appropriate formulation and pH and other processes, although the steel parts can not produce obvious replacement copper in the plating solution, the binding force is still unreliable, and the fundamental reason is that it is very difficult to activate the surface of the substrate quickly. It is imperative to replace pre-plating and realize cyanide-free pre-plating of copper. There will be some further discussion on how the conditions for achieving this purpose can be achieved.

Keywords: binding force; Replacement copper; Passivation; Activation; Pyrophosphate copper plated

Medium figure classification number: TQ153.14 Document identification code: B

Keypoints of Weak Adhesion of Copper Coating on Iron and Steel

Articles Direct Plated in Pyrophosphate Bath

YUAN Shi-pu

0 Introduction

Copper pyrophosphate plating was introduced in 1847. The formula of 1940 is close to modernity, and around 1950 it was used for anti-carburizing and anti-hydrogen embrittlement of steel parts, and its many advantages have attracted people's attention. Kaoru Aoya[1] and the domestic Chu Rongbang have discussed the process in more detail. Over the decades, many experimental studies have been done on steel parts to directly plate copper in copper pyrophosphate solution, but they have failed. At present, it is only used for thickening copper plating after cyanidation pre-copper cyanide of zinc die castings, while steel parts are pre-plated with copper cyanide or dark nickel and thickened with sulfate bright acidic copper plating. Poor binding force is always a difficult point for the process to achieve direct plating, and there has been no substantial breakthrough, otherwise it has long been cyanide-free in pre-plating (hereinafter referred to as pyrophosphate copper plating for coke copper).

The journal published an article entitled "The Effect of pH on the Copper Plating Adhesion of Pyrophosphate copper plating of steel matrix" in the 10th issue of 2008[2], arguing that the poor binding force is caused by the replacement copper generated on the steel parts, as long as the pH is controlled between 8.0 and 8.5, so that the stable potential of copper and iron is close, no replacement copper will be generated, that is, good binding force can be obtained; If the pH is higher than 8.5, the coating crystallization is coarse and the binding force is poor due to insufficient cathode polarization. This is contrary to the research results of most electroplating workers for many years, and they dare not agree.

1 Susceptibility to passivation of iron

In fact, the pH of the copper process is between 8 and 9, the pH is lower than 8, the brightness is better, but the binding force and dispersion capacity are reduced; the pH above 9 is low, and even the precipitation is generated. Literature[1] discussed in more detail. But why are steel parts coated with coke copper for decades, and the adhesion is not good? Not the effect of pH. The fundamental reason is that the copper layer is not plated on the surface of the activated steel matrix, but obtained on the passivation layer, so the binding force is not good.

Iron is a more passivation metal (although its passivation is not as good as metals such as titanium, chromium, aluminum, nickel, molybdenum and beryllium). It has been tried to break the wire in the air and immediately put it into mercury, the section can not generate iron amalgam, and only by burying the wire in mercury and breaking, the section due to the isolation of the air is not oxidized to form a FeO passivation layer, in order to generate iron amalgam. When the steel parts are plated, when the pretreatment is degreased, activated and then washed, the dissolved oxygen in the water is enough to passivate its surface to varying degrees. Activated iron will passivate in distilled water, and its blunt state can be maintained for a long time under alkaline conditions.

In the normal coke copper plating solution, the phenomenon of replacement copper on the steel parts is not observed. Copper, lead and tin are metals that are not easily passivated. Whether it is pre-plated copper or has tried to use propylene-based sulfur as a blocker to dip copper and then plate copper, it is the use of copper is not easy to passivate, copper is re-plated, the two layers of copper between the adhesion force is better.

2 Conditions for direct copper plating of steel parts

To achieve good adhesion to direct copper plating of steel parts, two conditions must be met at the same time: 1) no rapid replacement copper (slow displacement copper layer when copper immersion may also have a good adhesion force; 2) the surface of copper electrodeposition must be an activated metal. Both are indispensable.

Using a variety of alkaline copper plating processes of the coordination agent, as long as the coordination agent (single or composite) is properly selected and the mix ratio is high enough, it can not produce replacement copper, and the replacement problem is easy to solve, but it is not easy to make the steel parts quickly activated. There have been many experimental studies on this. In the 20th century, when the cyanide-free electrotechnology was tackled, the potential-time curve on the clean steel specimen in a variety of plating solutions was measured, and it was found that in addition to the potential of the cyanide copper plating solution gradually becoming negative with time to reach a stable value, the potential in all cyanide-free alkaline copper plating solutions also reached a stable value with time, and the potential in the coke copper liquid was the most positive (the passivation was the most severe). It also discusses the conditions for direct copper plating of steel parts.

3 Ways to revitalize steel parts

3.1 Chemical activation

So far, the only substances that can chemically activate steel parts are H+ and CN-. The chemical activation of H+ is equivalent to pickling for copper plating and can only be acidic or slightly acidic. However, under acidic conditions, it is impossible to find a coordination agent with strong coordination ability for Cu2+, the activity of Cu2+ is high, the replacement phenomenon must be serious, and the steel parts cannot be directly plated with copper. Complex plating must be used under alkaline conditions, otherwise Cu(OH)2 precipitation will be generated, and only CN- has a chemical activation effect on passivated steel parts, which brings great difficulties to the cyanidation of copper plating. Over the years, coupled with the "squeamish" nature of the process and the sensitivity to impurities, the successful industrial application of the process is very small. The pre-immersion activation method of engorged non-cleaning into the tank after immersion in organic acid activation is intended to maintain the activated state of alkaline cyanide-free copper plating, but there are also many problems in industrial applications that are difficult to solve.

3.2 Electrolytic activation

When flash plating treatment in cyanide-free alkaline copper plating solution with high mix ratio and low main salt concentration, the strong reducing ability of the active hydrogen atoms of hydrogen evolution under low cathodic current efficiency can activate the chemical parts, remove the passivation film, and realize activated electrodeposition, the reaction formula is as follows:

H++e-→H FeO+2H→Fe+H2O

Such as: 28gL copper pyrophosphate, 254g/L potassium pyrophosphate, 23gL potassium citrate, pH of 8.0 ~ 8.3, temperature of 18 ~ 23 °C, strong air stirring under the cathode current density of 2.0 ~ 2.5A / dm2 under the flash plating 2 ~ 3min [1], and then thick copper plating. The flash plating process is a "two-step method", and a set of electrolytic equipment needs to be added.

3.3 Potential activation theory

Professor Feng Shaobin has done a lot of research on potential activation theory over the years, which believes that when the cathode current density reaches a certain critical value, when the cathode potential is negative at a critical activation potential, the iron part is suddenly activated and the activation electrodeposition can be realized, and the binding force is good. Chen Tianyu once cited the data provided by Professor Feng Shaobin in his early years, and the critical activation current density of coke copper is 2A/dm2. Professor Feng said at the Electroplating Technology Seminar held in Guangzhou in November 2008 that the current values are lower. The actual critical activation current density value is not a fixed number, which is affected by various factors such as the type of coordination agent, the hooking, the mix ratio, the pH and the liquid temperature. The potential activation theory only explains the possibility of activated electrodeposition from the electrochemical epigenetic phenomenon, and does not reveal that the passivation layer is removed

Chemical reaction substance. When the critical activation potential is estimated, the following reaction occurs:

FeO+H2O+2e-→F+2OH-

Potential activation theory is used in industrial production, the problem to be solved is how to ensure that when the deep recess of the workpiece reaches the critical activation current density when the workpiece is plated, the sharp corner of the workpiece will not be burned, that is, the critical activation current density should be very small, and the process allows JK to be large, and the specific work is a lot, not to be described.

4 Special problems of the coke copper process

1) The final stable potential of the steel specimen in the coke copper liquid is corrected compared with many other cyanidine-free copper, indicating that the passivation is more serious. Ge Peiran [4] believes that the reason is that the steel parts also form some form of phosphating film in the coke copper liquid, and the addition of oxalic acid can dissolve the phosphating film. After adding 20 to 30g/L oxalic acid, the steel parts were activated, but the phenomenon of replacing copper was produced.

2) The role of ammonia, coke copper liquid must be more coordinated, and NH3 is indispensable. NH4+ introduced in the form of ammonium citrate is also converted to NH3 under alkaline conditions, and NH3 is constantly volatile under alkaline conditions. Literature[1] The effect of ammonia has been discussed in more detail: the light range of the layer is very narrow when the ammonia is not added to the plating solution, the junction is rough and the anode is poorly dissolved, and when it is too much, the bright area is whitish and foggy, and the dispersion ability is reduced. When the ammonia range is 2~ 5mL/L, 0.06mL/hL should be added, and it should be added frequently to keep the plating solution slightly ammonia-flavored. At high pH, the coating crystals are coarse, which may be related to excessive ammonia volatilization loss.

3) Pyrophosphate copper plating also has hydrolysis to produce too much orthophosphate, so that the cathode current efficiency decreases, conductivity decreases, the brightness range narrows and the coating fringes (but the dispersion capacity rises) problems, the new liquid JK upper limit can reach 3A/dm2, but the aging solution is only about 0.8A/dm2, the wastewater contains phosphorus, ammonia and nitrogen and is difficult to deal with. This article is not intended to be discussed.

5 Discussion

1) Literature [2] JK = 1A / dm2 constant current density coated coke copper single layer copper assessment method is not strict, should be acidic copper thickened and then plated bright nickel after the adhesion force assessment, it is more in line with the production reality. The reason is that the ductility of the single thin copper layer is good, and it is not easy to see the problem.

2) If judging from the stable potential, the pH is less likely to produce replacement copper at 9 to 10, and the binding force should be better, but the test results are not good, so it can be seen that the root cause of poor binding is not the replacement copper.

3) The good binding force at a pH between 8.0 and 8.5 may be just a coincidence, and the root cause should be that under the pH condition of the formulation, according to the potential activation theory, its critical activation JK is less than 1Adm2.

4) If the 250mL Hall tank is used, the 1A current stir plating for 20min, when the pH is 8.0 ~ 8.5, the entire specimen is bent and broken along the length direction or a thermal shock test, the binding force in the whole range is good, and the conclusion that the adhesion force is good when the pH is appropriate can be reached. The key is not in pH, but in the rapid activation of the matrix.

bibliography

[1] Aoya Ryo, Imai Yuichi, Kawai Kei Metal Sokki Technology[M].东kyo: 桢书, 1974.74.

Wang Longsheng,Wang Chunxia,Li Ganhu. Effect of H on copper plating adhesion of pyrophosphate in steel matrix[J].Electroplating and Finishing,2008,30(10):34-36.

[3] Bonding force of copper plating of yuan shipu steel parts[J].Material Protection,1981,14(2):28-32

Application of ge peiran high stability pyrophosphate copper plating in our factory[J].Electroplating and Environmental Protection,1987,15(6):30-33.