JP4672309B2 - Alkaline zinc plating method on cast iron - Google Patents

Description

  The present invention relates to a method for applying zinc-based plating to cast iron using an alkaline zinc-based plating bath.

Electro-galvanizing plating on cast iron is a technology required in fields such as automotive brake parts and piping joint parts for building materials. Generally, acidic zinc-based plating baths (ammonium chloride bath, potassium chloride bath, sulfuric acid) There are few examples of plating from an alkaline zinc plating bath at a practical level. This is because, when an attempt is made to plate cast iron from a conventional alkaline zinc plating bath, most of the current is consumed for electrolysis of water, and zinc is electrodeposited only slightly on the high current density portion. It is because it does not.
However, acidic zinc plating baths have poor wastewater treatment performance and high processing costs. In plating from acidic zinc plating baths, the plating film thickness varies greatly depending on the portion of the object to be plated. The inner surface, recesses, holes, holes, etc.) are likely to rust, and new problems include the inferior corrosion resistance of trivalent chromate coatings on galvanized plating compared to those plated from an alkaline bath. Therefore, there has been a demand for an alkaline plating bath for zinc-based plating on cast iron.

When the cast iron is plated from the alkaline zinc plating bath, little or no zinc electrodeposition occurs at the normal plating current density (about 1 to 3 A / dm ² ). When a several times higher current density load is applied, zinc electrodeposition can be obtained in part, and the obtained electrodeposited zinc is rough plating and practical smoothness cannot be obtained.
No attempt to solve such a phenomenon of galvanization from an alkaline bath to cast iron can be found from the patent literature and other technical literatures related to galvanization so far.
Then, this invention makes it a subject to electrodeposit zinc from an alkaline bath and to make the electrodeposition zinc into a smooth plating film instead of rough plating.

As a result of intensive studies to solve this problem, the present inventors have tried to plate cast iron from an alkaline zinc-based plating bath, and a continuous large amount of hydrogen gas due to electrolysis of water at the interface between the plating solution and the object to be plated. It has been found that this hydrogen gas covers the object to be plated, shuts off the supply of zinc ions to the interface, and the plating electrodeposition is extremely hindered.
Therefore, the hydrogen gas blocking the supply of zinc ions to the interface between the plating solution and the object to be plated (hereinafter referred to as “electrodeposition surface”) is separated from the interface by physical means (hereinafter referred to as “hydrogen gas removal”). ), It is considered that the electrodeposition of zinc is facilitated by promoting the supply of zinc ions to the electrodeposition surface, and the flow or vibration of the plating solution and / or the vibration, vibration or When rotation or the like was performed, the hydrogen gas adhering to the interface was removed, the supply of zinc ions to the interface of the object to be plated was promoted, and galvanization could be easily performed.

However, in the conventional alkaline zinc plating bath with soluble zinc as the anode, if hydrogen gas is removed by the flow of the plating solution or the rocking of the processed material, it adheres to the surface of the zinc anode or settles at the bottom of the plating tank. The anode slime that is floating floats in the plating solution, contaminates the plating solution, and adheres to the plating film, resulting in defective plating. Alkaline zinc plating on general steel products often has an anode back (cloth bag) on the anode zinc, but in the case of plating on cast iron several times to several tens of times compared to plating on general iron products Since the anode current density is high, a large amount of anode slime is generated, the cloth bag is severely clogged, the conductivity is deteriorated, the management of the plating bath is difficult, and it is not practical.
In order to avoid such contamination of the plating solution due to the anode slime, the present inventors adopted an insoluble anode in which no anode slime is generated in part or all of the anode, and the zinc ions are supplied from other than the anode. This reduces or eliminates the negative effects of anode slime, and removes hydrogen gas from the surface of the object by freely changing the flow or vibration of the plating solution, or the strength, direction, etc. of rocking, vibration or rotation of the object to be processed. It became possible to perform electrodeposition on cast iron from alkaline bath zinc-based plating.

  Electrodeposition can be promoted by increasing the zinc ion concentration in the plating bath or by increasing the plating bath temperature, but this acceleration effect becomes significant when hydrogen gas is removed. If not, it is a little.

（式中、ａ及びｂは２〜４の整数であり、
ｍは０〜６の整数であり、
ｎは１以上の整数であり、
Ｘは有機又は無機イオンの残査であり、
Ｙは硫黄又は酸素原子である。）
をもつ化合物を用いることで、この課題が解決できることを見い出だした。 In this way, it became possible to electrodeposit cast iron from an alkaline plating bath. However, an additive (brightener) to a conventional alkaline zinc plating bath, for example, an aliphatic amine such as imidazole or polyalkylene polyamine, Use of additives mainly composed of reaction products of epihalohydrin causes problems such as rough plating (bark-like plating, generally referred to as “burning”) and deterioration of additives at plating solution temperatures of 30 ° C. or higher. occured.
We have the structural formula as an additive (brightener) to the plating bath:

(Wherein, a and b are integers of 2 to 4,
m is an integer of 0-6,
n is an integer of 1 or more,
X is the residue of organic or inorganic ions,
Y is a sulfur or oxygen atom. )
It has been found that this problem can be solved by using a compound having.

  As a result, it became possible to perform electrodeposition of plating from an alkaline zinc plating bath to cast iron. However, as a new issue, the alkaline solution that entered the plating process in the cast iron “s” was removed after plating. There were problems such as oozing out on the surface for several hours to several days, causing discoloration or corrosion of the conversion coating such as chromate on the surface of the galvanized surface. As a means for solving this problem, the object to be plated (cast iron) subjected to alkaline zinc plating is repeatedly subjected to a temperature difference such as hot water washing and water washing in the washing process after plating, and “swell” is made by utilizing the expansion and contraction of the object to be plated. The problem was solved by washing out the alkaline solution in the water.

  As described above, applying zinc-based plating from an alkaline bath to cast iron has caused various problems in addition to simply electrodepositing zinc-based plating on the surface of the object to be plated, since there is no experience in practical use so far. However, the present inventors solved these problems one after another, established a practical alkaline zinc plating method, and reached the present invention.

  By using the present invention, it has become possible to perform electrodeposition of zinc from an alkaline plating bath to cast iron, which has been considered difficult until now, and further, the electrodeposited zinc can be made into a smooth plating film instead of rough plating. Become.

Next, the present invention will be described in detail.
As a means for plating cast iron from an alkaline zinc-based plating bath, it is possible to remove hydrogen gas by continuously or intermittently separating hydrogen gas generated on the electrodeposition surface from the electrodeposition surface as soon as possible. This is one aspect of the invention.

The flow of the plating solution performed as the hydrogen gas removing means can be performed by stirring the plating solution, a jet, a tank flow, or the like.
The flow method of the plating solution is not particularly limited. Specific examples include screw stirring, air stirring from the lower part of the processing object, discharge of plating solution from the lower part or side of the plating tank, plating solution in the plating tank. There is a flow in one direction (flow in the tank), ultrasonic waves from the lower part or side surface of the plating tank, and the effect is increased by combining two or more of these.

The plating solution can be vibrated by ultrasonic vibration, low frequency vibration or the like. The direction in which the workpiece is swung is not particularly limited. For example, the vertical direction, the front-rear direction, the left-right direction, and the like are possible. The workpiece can be vibrated by a vibrator or the like, or the workpiece can be rotated. Plating is also effective. Moreover, the effect increases by combining two or more of these means.
Conventionally, a method of rotating an object to be plated such as a barrel plating method (rotary plating) is known. However, in the barrel plating method, the rotation speed is slow (about 5 rpm), and hydrogen gas removal at the interface of the object to be plated is performed. Since it is rarely done, it is essentially a different method from the present invention. Furthermore, barrel plating has a low cathode current density due to restrictions on the power supply method. Even if the cathode current density is increased, there is no brightener that can withstand this, and the problem of the present invention cannot be solved.

  It is difficult to limit the preferred flow method and rocking method because the shape of the processed material and the quality requirements of the effective plating surface are different. However, the flow and vibration of the plating solution are directional and uniform over the entire processed material. Since the effect of removing hydrogen gas cannot be obtained, for example, “tank flow” and “intermittent discharge of plating solution” are combined, and further, two or more types are supplemented, such as “oscillating or rotating the workpiece” It is preferable to remove the hydrogen gas as uniformly as possible by combining the two.

Removal of hydrogen gas from the electrodeposited surface raises dirt such as the anode surface of the zinc anode and the anode slime at the bottom of the plating tank and floats in the plating bath, causing plating defects. In order to avoid the adverse effect of the anode slime floating, in the present invention, a part or all of the anode is made an insoluble anode, and the whole is preferably made an insoluble anode.
Insufficient zinc ions in the zinc-based plating bath due to the use of an insoluble anode is preferably provided by providing a zinc dissolution tank separately from the plating tank and circulating the plating solution between the plating tank and the zinc dissolution tank.
The zinc dissolution method in the zinc dissolution tank includes a method in which zinc is dissolved by bringing metal zinc and a metal noble from zinc into contact with each other in a plating solution, and further zinc is accelerated by applying an external voltage. Some proposals have been made, such as a method of causing the dissimilar metals to be displaced by swinging or rotating, but the melting method is not particularly limited in the present invention. In general, there is a method in which a zinc plate is placed in an iron cage in a dissolution tank, zinc ions are supplied to the plating solution, and the supplied plating solution is returned to the plating vessel (preferably through a filter).
If there is no place to install a zinc dissolution tank, zinc ions are dissolved by contact with dissimilar metals, such as by immersing an iron basket containing zinc in a plating bath in an open space in the plating tank. It can also be supplied to the plating bath. If the means is not sufficient to supply zinc ions, a part of the anode can be supplemented with a soluble zinc anode. However, in this method, since plating on cast iron is plating with a high current density, anodization occurs rapidly, current does not flow easily, and negative current distribution is also adversely affected. When zinc is dissolved in the plating tank, the anode slime is plated by a method such as mounting an anode back or a zinc ion permeable diaphragm equivalent between the zinc to be dissolved and the cathode (material to be plated). It is important not to float in the bath.

  Moreover, the removal of hydrogen gas from the cathode and the use of an insoluble anode according to the present invention are not only for the purpose of promoting plating electrodeposition and avoiding the negative effects of anode slime, but also for the high current density plating work required for plating cast iron. It has the effect of greatly reducing the high bath voltage involved, and is also effective for energy saving measures. In particular, the above hydrogen gas removal means has the effect of removing not only the hydrogen gas on the electrodeposited surface of the cathode but also the oxygen gas on the anode surface, and the electric resistance of the plating bath is lowered, so the plating bath voltage is greatly reduced. Can do. Further, it is possible to further reduce the voltage by increasing the anode area by providing irregularities on the anode using a metal that is easy to process into an insoluble anode material, such as a steel plate. It is thought that the amount of hydrogen generated by the voltage drop can also be reduced.

  The insoluble anode material that can be used in the present invention is not particularly specified as long as it does not adversely affect the zinc-based plating, but generally does not dissolve by anodic electrolysis in an alkaline plating bath such as iron, nickel, cobalt, titanium, and carbon. Insoluble conductive material plating such as nickel plating on iron and cobalt plating or iridium oxide coating on titanium can also be used.

As mentioned above, when applying zinc-based plating to cast iron using an alkaline bath, providing a hydrogen gas removal means is effective in promoting electrodeposition, but as a factor that further facilitates electrodeposition, There is a zinc ion concentration in the plating bath and the temperature of the plating bath. The higher the zinc ion concentration in the plating bath and the higher the plating bath temperature, the more zinc ions are supplied to the electrodeposition surface, and the easier the electrodeposition is. . And this electrodeposition acceleration | stimulation effect raises significantly by using together with the hydrogen gas removal of a cathode. However, when the concentration and temperature are increased, there is a problem that the additive is liable to be altered due to the strong alkaline plating bath.
For example, an additive mainly used in a conventional alkaline zinc plating bath, that is, an additive mainly composed of a reaction product of an aliphatic amine such as imidazole or polyalkylene polyamine and epihalohydrin is used for alkaline zinc plating on cast iron. When it is used, since it is a high current density plating operation (4 to 40 A / dm ² ), even if the zinc ion concentration is about 15 g / L, the high current density portion of the object to be plated becomes a bark-like rough plating (discoloration), and the zinc ion concentration It was found that the higher the value, the worse. Further, it has been found that when the plating bath temperature is set to 30 ° C. or higher at a caustic soda concentration of 140 g / L in the plating bath, the additive is altered and the glossy effect is lost. Thus, it has been found that the use of the conventional additive has a practical problem.

（式中、ａ及びｂは２〜４の整数であり、
ｍは０〜６の整数であり、
ｎは１以上の整数であり、
Ｘは有機又は無機イオンの残査であり、
Ｙは硫黄又は酸素原子である。）
をもつ化合物が有効であることを見い出した。
この化合物の構造式において、ｎは重合度を示し平均６程度が最適であり、小さいと高電流密度部が粗めっきとなりやすく、大きいと電流効率が低下し電着しにくい傾向にある。
Ｙは硫黄又は酸素であり、硫黄の場合は特に光沢性に優れ、酸素の場合は電着物の二次加工性に優れている。またａ及びｂは２〜４であるが、小さい方が低電流密度部の光沢が良く、電着速度も速い。また大きい方が均一電着性に優れ、高電流密度部が粗めっきになりにくいため、被めっき物の用途、要求度に応じて選択できる。または、パラメーターの異なるこれらの化合物を組み合わせることにより複数の効果を同時に発現することもできる。 The present inventors examined not only a bark-like rough plating (discoloration) even at a high zinc ion concentration but also a new additive with little alteration in a high-temperature plating bath, and found that the caustic soda concentration of the plating solution was 90 g / Structural formula as an additive capable of supporting a wide range of zinc ion concentration of 10 to 65 g / L and plating bath temperature of 15 to 70 ° C. at L to 160 g / L:

(Wherein, a and b are integers of 2 to 4,
m is an integer of 0-6,
n is an integer of 1 or more,
X is the residue of organic or inorganic ions,
Y is a sulfur or oxygen atom. )
It has been found that a compound having a value is effective.
In the structural formula of this compound, n represents the degree of polymerization, and an average of about 6 is optimal. When it is small, the high current density portion tends to be rough plating, and when it is large, the current efficiency is lowered and electrodeposition tends to be difficult.
Y is sulfur or oxygen. In the case of sulfur, the gloss is particularly excellent, and in the case of oxygen, the secondary workability of the electrodeposit is excellent. Moreover, although a and b are 2-4, the one where it is small has good gloss of a low current density part, and the electrodeposition rate is also quick. Moreover, since the larger one is excellent in throwing power and the high current density part is less likely to be rough plating, it can be selected according to the use and requirement of the object to be plated. Alternatively, a plurality of effects can be expressed simultaneously by combining these compounds having different parameters.

The additive concentration in the plating solution is 0.5 g / L to 20 g / L, but 1 g / L to 4 g / L is preferable in consideration of plating stability and economy.
This additive enables plating conditions with a plating solution having a caustic soda concentration of 140 g / L or more and a temperature of 30 ° C. or more, which is a plating solution condition in which a conventional additive is decomposed. Alkaline zinc plating on cast iron at a speed superior to acidic zinc plating by plating under conditions of ² g / L, caustic soda concentration 150 g / L, zinc ion concentration 55 g / L, plating bath temperature 50 ° C., cathode current density 15 A / dm ² It was realized.
The plating bath concentration of the present invention will be described in detail. The caustic soda concentration maintains the conductivity of the plating solution and enhances the solubility of zinc ions. The higher the zinc ion concentration, the higher the caustic soda concentration must be. In the present invention, it is 90 g / L to 160 g / L, but in general, the zinc ion concentration is 10 g / L, the caustic soda concentration is about 100 g / L, the zinc concentration is 25 g / L, the caustic soda concentration is about 140 g / L, and the zinc concentration is 50 g / L. The zinc concentration is the most influential factor in the plating electrodeposition rate, uniform electrodeposition, and rough plating of the high current density part. The material, shape, and plating requirements of the object to be plated The zinc concentration of the present invention is 10 g / L to 65 g / L, and usually the higher the zinc concentration (30 g / L) because the plating electrodeposition speed is important. Generally, the above is selected. However, since the upper limit of the plating film thickness is limited depending on the use of the object to be plated, it is possible to reduce the zinc concentration at the expense of the plating speed.

The plating solution temperature shows a tendency similar to the zinc concentration, and the higher the temperature, the faster the plating speed, the lower the electrodeposition, and the higher current portion rough plating is likely to occur. Therefore, in order to perform plating according to the requirements of the object to be plated, the zinc concentration cannot be changed abruptly, and therefore it is possible to adjust the plating solution temperature. The plating temperature of the present invention is 15 ° C. to 70 ° C., and generally about 40 ° C. to 50 ° C. is appropriate.
Further, the cathode current density can be used at 4 A / dm ^{2 to} 40 A / dm ² , and can be selected together with the zinc concentration and the plating solution temperature according to the required degree of the object to be plated, but generally 10 A / dm ^{2 to} 25 A. / Dm ² . For the purpose of high-speed plating, a high current density of 30 A / dm ² or more is possible under strong plating solution flow, high zinc ion concentration, and high plating solution temperature.

  Cast iron has a liquid-permeable hole called “su”, and the processing chemicals contained in this “su” during processing steps such as alkaline degreasing (containing surfactants), acidic derusting, and plating. Oozes out over several days, causing a decrease in corrosion resistance and discoloration of the chromate conversion coating, which is the final step of galvanization. In order to avoid this, a temperature difference is given in the washing process after plating, and the chemicals in the soot can be removed by utilizing the expansion and contraction of the cast iron. As a means for giving the temperature difference, for example, a method of performing hot water washing and water washing by shower or dipping can be mentioned. It is not necessary to lengthen the cleaning time, and it is sufficient that one process takes 5 to 10 seconds or more for each temperature. It is preferable that the temperature change from the low temperature (high temperature) cleaning to the high temperature (low temperature) cleaning is counted once and the temperature change is repeated twice or more. The larger the temperature difference between the low temperature cleaning and the high temperature cleaning, the better. It is also effective to use ultrasonic waves in combination with this cleaning process, but the effect is further increased by giving a temperature difference.

  The alkaline zinc plating in the present invention is alkaline zincate zinc plating or alkaline zinc alloy plating (zinc-iron, zinc-iron-cobalt, zinc-cobalt, zinc-nickel, etc.). Since hydrogen gas removal, zinc ion concentration change, and plating solution temperature change affect the alloy ratio, it is necessary to set plating conditions anew.

  So far, zinc-based plating on cast iron has been performed with an acidic bath, but the present invention enables practical use of zinc-based plating from an alkaline bath. The alkaline bath is an environmentally friendly plating bath that is easier to treat wastewater than the acidic bath, and has higher corrosion resistance than the acidic bath, and has advantages such as uniform electrodeposition and film properties. It is possible to provide a high-quality cast iron zinc-based plated product.

  The present invention will now be described in the following examples, which are provided to better understand the present invention and its advantages and are not intended to limit the invention.

Plating test Zinc plating was performed on cast iron for the examples and comparative examples under the following plating conditions. The results are shown in Table 1.

Workpiece: Spheroidal graphite cast iron flat plate (100mm x 100mm x 8mm)
Pretreatment of plating: alkaline degreasing → water washing → hydrochloric acid pickling → water washing → alkali neutralization → plating
(General plating pretreatment was used.)
Additive of the present invention:
Additive (1) Structural formula a = 2 b = 3 Y = O 2 g / L
Additive (2) Structural formula a = 3 b = 3 Y = S 2 g / L
Conventional additives:
Additive (3) JASCO H-1218 (manufactured by Nippon Surface Chemicals) 6 ml / L (Aqueous solution mainly composed of reactants of imidazole, aliphatic amine and epichlorohydrin as a brightener for alkaline zinc plating)
Plating bath composition: Zinc ions 20 g / L, 55 g / L,
Caustic soda 150g / L,
Plating bath temperature: 25 ° C, 50 ° C
Anode: Distance between iron plates: 10-15 cm (varies due to cathode swing)
Cathodic oscillation: Amplitude 4-5 cm in the direction of anode 60 reciprocations / minute Plating time: 10 minutes Plating film thickness measurement position:
A: Surface of the object to be plated 15 mm inside from the upper right corner to the diagonal direction when viewed from the anode side (anode side)
B: Plated object surface (anode side) center
C: Criteria for judging the center of the back side of the object to be plated ◎: Practically satisfactory
○: Satisfied
Δ: Practical depending on the requirement of the object to be plated
×: Not practical

Claims (7)

In this method , the zinc plating is performed directly on the cast iron. The plating bath is an alkaline zinc plating bath containing zinc ions and alkali hydroxide, and part or all of the anode is an insoluble anode. A plating method characterized by supplying a part or all, and continuously or intermittently flowing or vibrating a plating solution and / or swinging, vibrating or rotating an object to be plated.   The method for plating cast iron according to claim 1, wherein zinc ions are supplied to the plating tank by circulating a plating solution between the plating tank and a zinc dissolution tank provided separately from the plating tank. The caustic soda concentration of the plating bath is 90 g / L to 160 g / L and / or the zinc ion concentration is 10 to 65 g / L, and / or the temperature of the plating bath is 15 to 70 ° C. and / or the cathode plating method to cast iron according to claim 1 or 2 current density is ^{4A / dm 2 ~40A / dm 2} . The additive to the plating bath containing 1 or more types of the compound represented by the following structural formula used for the plating method as described in any one of Claims 1-3.

(Wherein, a, b: integers of 2 to 4 m: integers of 0 to 6 n: integers of 1 or more X: residue of organic or inorganic ions Y: sulfur or oxygen)   The method for plating cast iron according to any one of claims 1 to 3, wherein the additive according to claim 4 is used as an additive to the plating bath.   A method for plating cast iron, further comprising a step of cleaning the object to be plated by applying a temperature change twice or more after plating by the plating method according to any one of claims 1 to 3 or 5.   A zinc-based plated product of cast iron plated by the plating method according to any one of claims 1 to 3, 5, or 6.