Feng Qiaoyu  Pan Guokuan  Zhang Chunhe

Guangzhou Mateys Chemicals Ltd.

Abstract: Zn-Ni alloy has pretty high anti-corrosion skill and perfect machinability which reflectsa good market outlook. It is widely used in the auto, the aviation, the machinery and the electronics.Follow us to know its specifications, technology and operationsn Key words: Zn-Ni alloy, plating

0  Introduction

Since its advent, Zn-Ni alloy technology has caught widespread attention in plating industry becauseof its excellent anti-corrosion skill. But for some limited applications, it still cannot be widely applied.During a long time, Zn-Ni alloy technology is just used within academic research. While the worldwide co-operationand continue further study keeping on, operation maintenance has been improved and plenty of production applications have been solved. It was developed and applied into the auto, the aviation, the machinery and the electronics industry.

With the help of its special anti-corrosion skill and high wear resistance, the auto industry has been booming in the recent years. Earlier in 1980s, it has been seen in the industry line abroad. Meanwhile, it was far behind inChina, which been first reported in 1990s, until recent year to produce in the industry. But small problems still discourage production, only a few factories can handle the  bulk production. After study on continue trial tests and proper bulk production tests inChina, kinds of manufacture-related problems have been solved already.

Honestly speaking, Zn-Ni alloy technology is now stable and good. It has perfect anti-corrosion, easy operation, easy maintenance and quick passivity, which are our advantages ahead of others in this industry.

1、 Perfect anti-corrosion skill

Firstly, Zn-Ni alloy deposit will be contrasted to Zinc deposit. For example, in picture 1, we have experimental installation, rack, columned iron bar, 5 μm thickness, and follow plating tests in 3  situations. TestⅠ, heat to follow neutral salt spray test.  TestⅡ, do salt spray test after kinds of passivity. Test Ⅲ,do salt spray contrast tests with the change of nickel ions.

Through salt sprays’ observation and records, we can conclude to the following histogram. In Picture 2,  it is the comparison of Zn and Zn-Ni alloy in TestⅠ.

Picture 2、

From the above chart, we can see its greatest feature is to keep anti-corrosion performance against extremely bad after being heated. In the neutral salt spray test, salt mist is several times than one in Zinc plating. After some colors passivate, its anti-corrosion performance becomes better.

Picture 3、after passivation, Zn-Ni alloy in neutral salt spray test.

From the above chart, we can see the green appears best compared with other colors passivation. While black and colorful come next, the rest chrome-free with white rust is the worst.

Picture 4、 Salt spray contrast tests with the change of nickel ions.

From the above chart, when nickel ions increases, it gets good performance between 7.5% and 10%, and seldom affect to the anti-corrosion. But if the nickel ions goes higher than 12%, it will be hardly to form the passivate film, then bad to the anti-corrosion performance. So, in daily production, nickel ions is a key part, which should be controlled between 6-12% to get the best effect. 

2、Composition

2.1、Composition：

Zinc chloride(ZnCl2)	60-80g/L
Ammonium chloride(NH4Cl)	180-200g/L
Nickel chloride(NiCl2)	50-150g/L
Startup	60-70ml/L
Complexing agent	30-35ml/L
Brightener	50-60ml/L
pH value	5.0±0.2
Anode	1(Ni):2 - 3（Zn）
Temperature	30-35℃

2.2、Alkaline Zn-Ni Composition

Zinc oxide(ZnO)	8-15g/L
Sodium hydroxide(NaOH)	90-140g/L
Nickel replenishment	12-15g/L
Startup	60-70ml/L
Complexing agent	50-70ml/L
Brightener	5-8ml/L
Temperature	28±2℃
Anode	Ni-Fe plate or Iron plate

2.3、Differences between Acid Zn-Ni alloy and Alkaline Zn-Ni alloy

Alkaline Zn-Ni alloy plating strong points

1、Good uniform plating skill (dispersing skill) to get a uniform deposit

2、Slight differences of nickel ions in high current area and low current area

3、Perfect deep plating skill and excellent deposition skill

4、Stable solution, less deviation of all composition, high controllability

5、Stable nickel ions

Acid Zn-Ni alloy plating weak points

1、Comparatively poor uniform plating skill (dispersing skill) and uniformity

2、Great differences of nickel ions in high current area and low current area

3、Comparatively poor deposition skill in complex work parts

4、Great changes of solution, timely analysis and replenish on composition, especially pH

5、Great changes of nickel ions, timely control and replenish during production

Alkaline Zn-Ni alloy plating’s weak points

1、Low current efficiency at 50%-70% appr., low deposition speed at 0.25μm/min appr., especially in barrel plating

2、High operation cost

3、Slightly poor gloss and brightening speed

4、Fast corrosion speed of deposit in solution

Acid Zn-Ni alloy plating’s strong points

1、High current efficiency at 90%-95% appr., high deposition speed up to 1μm/min or above

2、Low operation cost, comparatively 50% appr. lower than alkaline Zn-Ni alloy plating

3、Significant gloss, wide range of adjustability, fast brightening speed

4、Slightly dissolve faintly acid of deposit

After years’ study and improvements, weak points of acid Zn-Ni alloy plating and alkaline Zn-Ni alloy plating become better. Based on work parts’ appearance and requirements, it can be chosen the right one in daily production. Generally, complex parts or high uniform requirements will take alkaline plating while columned or flat parts will take acid plating. If inner holes in parts or battery shells, it will take alkaline plating.

3、Composition effect:

3.1 Acid Chloride Zn-Ni alloy

A、Main salt: Zinc chloride(ZnCl₂) and Nickel chloride(NiCl₂)

Based on ammonium salt(NH₄ ion salt), Zn-Ni alloy solution has Nickel chloride(NiCl₂) and Zinc chloride(ZnCl₂) as previous choice, which offer the MUST metal ions to keep Zinc chloride within 50-90g/L. When it is low, dispersing skill and deep plating skill improve while current efficiency reduces; when it is too low, the deposit is easy to scorch, deposition speed becomes slow. But, when it is enough, the gloss enhances, dispersing skill reduces and current efficiency increases as well; when it is too much,

the deposit becomes rough and the gloss reduces. What is more, if Nickel chloride(NiCl₂) ions are low, anti-corrosion performance will be affected since electric potential differences cause low nickel in solution against co-deposition, while too much may cause poor passivate and anti-corrosion performance.

B、Conducting salt: Ammonium chloride(NH₄Cl)

In Chloride Zn-Ni alloy, Zinc chloride(ZnCl₂) is to enhance solution’s electro conductivity, reduce tank pressure, improve dispersing skill and coverage skill, get a certain complexing skill on Ni and Zn ions, stabilize solution, improve cathode’s polarization. All is directly affect on the deposit to make it compact and uniform. When it is too much, it may causes crystals separate out; when it is too low, it may affect electro conductivity, dark and rough in high area.

C、Additions: Mixed Complexing agents and Brighteners

In Chloride Zn-Ni alloy, kinds of additions may cause deposit bright and compact, enhance dispersing skill, balance Zn:Ni ratio, stabilize solution.

D、Weather and pH value

In Chloride Zn-Ni alloy, pH value is a key parameter. When pH value is too low, Zn anode dissolves too fast and brighteners easily to separate out, which may cause blur on deposit, too many Zn ions stimulate and Zn:Ni ratio disorder.

When pH value is too high, it may cause poor uniform skill; when it is in a certain high level, it may cause Zn ion hydrolysis, east to get burr and hydroxides, which may affect deposit performance.

The temperature is to take a significant effect on Ni ion of the deposit. When it is too high, Ni ions may increase; when it is too low, it cause perfect gloss but poor dispersing skill which may scorch.

3.2 Alkaline Zn-Ni Alloy

Main Salt: In Zinc oxide(ZnO) and complexing Nickel Mixture, it can be formed to Nickel hydroxid(Ni(OH)₂) precipitate in alkaline solution, which cannot join plating reaction. The addition of Complexing Nickel mixture can help solution stable, Ni ions stable, co-deposition. The high or low of Ni ions can affect the deposit Ni ion. The change of Zinc oxide(ZnO) may cause not only Zn:Ni ratio of deposit, but also deposition speed, gloss and dispersing skill.

Sodium hydroxide(NaOH): Sodium hydroxide(NaOH) is to enhance Zn complexing skill and electric conductivity, which help the anode dissolves uniformly. When it is excess, it can help enhance dispersing skill and reduce the resistance, while Zn anode dissolves too fast, it will cause Zn ions increases as well; when it is too low, it may cause Zinc hydroxide(Zn(OH)₂) precipitate, anode passivate and reduce dispersing skill.

Additives: Mixed additives. In alkaline plating, the great differences of Ni-Zn electrode may not help co-deposition or deposit forming. With the addition of complexing agent and brighteners mixture, it can force these two metal ions to keep complexing, and help similar electrodes or potentials join co-deposition to form a bright and uniform alloy deposit.

Temperature: It won’t take any significant change of temperature in Alkaline plating, while rough gains may be formed when in the high temperature.

Current density: The current in alkaline plating takes slight change of Zn-Ni ratio. Even if the current becomes bigger, nickel ions are stable between 6% and 10%.

4、Zn-Ni alloy Application and Operation  (Take Zhangzhou Yiquan Plating Factory as an example to discuss and get a conclusion of operation’s conditions)

4.1 Barrel Zn-Ni alloy plating：

Alkaline Zn-Ni alloy plating Work Flow:

Pre-treatment:

Grinding degreasing / shaking grinding degreasing → 3 times Wash Rinse → Acid derusting → 3 times Wash rinse → Weak alkaline activation

Post treatment:

→ Alkaline Zn-Ni alloy plating → Recycle → 2 times Wash rinse → Neutralization → 2 times wash rinse → Passivate → 2 times wash rinse → Wash rinse with hot water → Passivate → Spin & dry

Acid Zn-Ni alloy plating Work Flow:

Grinding degreasing / shaking grinding degreasing → 3 times Wash Rinse → Acid derusting → 3 times Wash rinse → Acid Zn-Ni alloy plating →Post treatment:

→ Recycle → 3 times Wash rinse → Passivate → 3 times wash rinse → Wash rinse with hot water → Passivate → Spin & dry

Rack Zn-Ni alloy plating:

Parts hanging → Ultrasonic wax release → 2 times wash rinse → Chemical degreasing → Ultrasonic degreasing → 3 times wash rinse → Cathode electrolysis degreasing → Anode electrolysis degreasing → 3 times wash rinse → Acid derusting → 3 times wash rinse → Weak alkaline activation → Alkaline Zn-Ni alloy plating → Recycle → 3 times wash rinse → Neutralization → 3 times wash rinse → Passivate → 3 times wash rinse → Wash rinse with hot water → Passivate → Dry

Remarks: Alkaline Zn-Ni alloy plating can be omitted in acid Zn-Ni alloy plating, even no neutralization to direct passivate.

From the above flow chart, we can see Zn-Ni alloy process is similar as Zinc plating process. Acid Zn-Ni alloy might be more simple since it can be directly passivate without neutral. While alkaline Zn-Ni alloy may need neutral since too much alkali left on the deposit, direct water rinse can remove it, so neutralization is chosen to help clean the surface and uniform the passivate.

5、Barrel alkaline Zn-Ni alloy’s application and operation

No matter acid or alkaline Zn-Ni alloy, rack plating will be more simple, which has better performance, current distribution, deposition speed and recycle exchange, esp. barrel alkaline Zn-Ni alloy. Because of alkaline functionality and deposit performance will be better and more stable, so most of domestic applications take it in actual production. But some problems still exist such as low current efficiency and low deposition speed, the deposit is easy to be corrupted by alkali and low efficiency during production. Now Mateys gets an conclusion of the most efficient ways to improve barrel plating effect and efficiency based on many actual operation examples.

A、Barrel improvement: Low alkaline Zn-Ni alloy , traditional barrels makes ions exchange lower, current distribution and efficient current lower and resistance larger. For example, in picture 1, traditional barrels, high voltage during production, low current, significant defects. In picture 2, improving barrels, low voltage during production, high current, no over-heat in the anode, deposition speed and production efficiency improvement.

   A、Barrels’ fast running in barrel plating: Traditional plating operation will be at 8r/min. Some kinds of sheet parts may be overlaying or stick on the barrel, which may cause defects, such as hole prints, yellowish, blacken. During actual operation, match with yarn mesh barrel, it can make barrel move faster at 12r/min, which can solve the above problems. See picture 3, the parts.

picture 3

B、Collect to add after barrel plating: Alkaline Zn-Ni alloy is easy to be corrupted in strong alkaline solution. From moving out of the tank to add additives, work parts are easier to be corrupted to cause yellowish, non-uniform or blacken. It can be used to solve the problems basically.

C、Neutral before passivate: Alkaline left on surface is hard to remove by water, attaching the surface which may cause non-uniform and defect in weak acid passivate. Neutralization should be similar as the gloss brought by Znic plating, which can remove the left alkaline, then follow to passivate a uniform deposit with proper concentration ratio, finally affect the color of the film. It is common to take attenuated hydrochloricacid(HCl) and attenuated Acetic Acid(C₂H₄O₂) ect.

6、Zn-Ni alloy passivate

Generally the gloss of Zn-Ni deposit is bright white with high anti-corrosion performance, but to be applied on different areas, the colors should be different. Passivate can solve this problem. At the same time, the deposit will significantly enhance its anti-corrosion. Zn-Ni deposit’s passivate is different from the traditional Zinc deposit because the deposit contains 8-12% Ni ion. With many years’ study, low Chrome or trivalent Chrome’s passivate has become stable.

7、Future

It is proved that Zn-Ni alloy is one kind of deposit with higher functions than Zn or Cd. Compared with the latter, it has a great anti-corrosion performance improvement. Not only for functional protective deposit, but also for decorative deposit. Be Cadmium replacement to avoid bad effect on the people and the environment, be Zinc replacement to hold better anti-corrosion skill with the same layer at 3-8μm than Zinc deposit. It is the ideal deposit with perfect quality to replace others.

With technology improvement and further study on Zn-Ni deposit, the performance will keep improving, the solution will be more stable and controllable and the passivate technology will be improved and trusted as well. We believe, in the near future, Zn-Ni alloy plating will be widely applied in the aviation, the auto, the machinery, the electronics and the military industry.