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Performance Characteristics of Zinc–Nickel Alloys and Dip-Spin Coatings


Toshiaki Murai

As the performance criteria for plated components have become more demanding, directives to meet strict environmental considerations have forced sweeping changes to surface finishes. The resulting field performance has been as varied as the alternatives. This article will compare zinc–nickel alloy plating and dip-spin coating technology with regard to various critical performance characteristics for automotive component applications. Corrosion resistance, total friction co-efficient, and contact with dissimilar metals, among other key properties, will be discussed.

In today’s competitive marketplace, consumers require higher-quality products than ever before. Automobiles can be exposed to severe conditions, especially in regions of North America and Europe. During the winter, municipalities spread salt and calcium chloride with sand for road safety. Sand particles impinge exposed surfaces, causing erosion and initiating surface cavities. Several zinc–alloy plating processes for automotive components that provide superior performance in highly corrosive environments are available on the market today. Following is a performance comparison between Zn-Ni alloy electroplating and dip-spin applications.

General Alloy Plating Mechanism for Corrosion Performance

More than 100 types of electroplated alloy processes have been invented since the zinc–copper alloy was developed in 1841. The merits of alloy plating are as follows:

  1. New phases that did not exist on metallography phase diagrams can be achieved.
  2. Homogeneous alloy compositions not attainable through standard melting methods, because low melting point metal vaporizes at the temperature of higher melting point metals.
  3. Thin film coating deposits can provide high-performance features:
  • Corrodes sacrificially to steel
  • Stability of corrosion by-products
  • Adherent conversion film
  • Low dissolution rate of conversion coating film in a corrosive atmosphere, such as salt solution

Corrosion of Steel Plated with Zinc and Zinc Alloy

Substrates are protected through the plating deposition of zinc and zinc alloys. Due to their poor ionization tendency, zinc and zinc alloys sacrificially dissolve prior to the substrate. As seen in Figure 1, electroplated zinc alloy deposits are superior and achieve a consistent stable corrosion performance.

The Reasons for Quality Corrosion Performance

As seen in Figure 3, the corrosion potential of a high Zn-Ni deposit is –0.9 to –1.0 V. It is closer to iron (–0.6 V) than zinc (–1.1 V). Therefore, the dissolution rate of high Zn-Ni is slower than a zinc deposit. The corrosion performance of a zinc–nickel plating film is a function of the nickel co-deposit percentage. It is commonly understood that zinc– nickel deposits have limits of nickel co-deposition (15%), at which the corrosion potential becomes noble. Zinc–nickel conversion coating technology is capable of achieving greater protection performance than that of a 16% of nickel co-deposit.

Furthermore, the crystal structure of the zinc deposit is a uniform single structure. The low Zn-Ni alloy crystal structure has two phases in its deposit with a galvanic cell corrosion mechanism. Refer to Figure 4 in which the low Zn-Ni has Zn, Ni3Zn22, d + ↓ as dual phases. The high Zn-Ni has only a Ni2Zn11 (↓) phase.

Finally, the corrosion by-products are very stable in a corrosive atmosphere (Fig. 5). The initial corrosion of the Zn-Ni alloy electrodeposit produces basic zinc chloride at the surface corrosion site, which acts as a barrier film with electric resistance and a reduced dissolution rate. The corrosion rates of the Zn, low Zn-Ni, and high Zn-Ni alloy deposits are compared in Table 1.

Table 1: Corrosion Speed
 ZincZn-7%NiZn-15%Ni
Metal density (ρ)7.147.267.40
Nyquist Prot charge transfer resistance (Ω)5005,00021,300
Corrosion current density (A)4.2×10-054.2×
10-06
9.8×
10-07
Corroding speed (µm/month)515.01.2

5% salt solution, immersion time 10 min.
Reference electrode: Ag/AgCl; Sub-electrode: Pt

 

Corrosion Resistance Performance

In Figure 6, a comparison of bending properties among three systems (alkaline zinc, low and high zinc–nickel is shown. For the high Zn-Ni system, a fine powdery deposit was formed by the bending test. However, no peeling off of the deposit was observed, and corrosion resistance at the bended area was intact. The bending test was executed using an 8-mm Erichsen test.

Bending Property/Ductility

Zinc–nickel alloy is harder due to its nickel content (Table 2). This higher hardness property provides superior anti-scratch performance. Corrosion performance was maintained after the severe bending test was performed (Figs. 6 and 7). The high nickel system exhibited a very minor increase in tensile stress.

Table 2: Deposit Hardness
Non-cyanide zinc100–140 Vickers
Acid chloride zinc60–80 Vickers
Zinc colbalt (acid)180–210 Vickers
Acind zinc nickel140–180 Vickers
Alkaline zinc-nickel250–310 Vickers
High zinc-nickel350–450 Vickers
Zinc-iron (alkaline)100–150 Vickers
Tin-zinc13–17 Vickers

 

Performance Comparison Between Zinc–Nickel Alloy and Dip-Spin

Both alkaline high zinc–nickel alloy and dip-spin provide a higher levels of corrosion protection with regard to salt spray protection. As shown in Figure 9, the advantage of using Zn-Ni is evident when the processes are subjected to cyclic corrosion testing (CCT). Many OEM groups consider CCT to be the proper representative of field performance.

The corrosion resistance advantage of zinc–nickel alloy is based on its electro-potential differential to steel. This results in a coating that is sacrificial to steel, thus protecting the functional performance of the part, which is paramount for fastener applications (Table 3).

Table 3: Comparison of Zinc-nickel Dip-spin (Descriptive)
 Film FormingPost-treatmentFilm StructureCorrosion Resistance Mechanism

High Nickel % Zn-Ni Alloy Plating

(High nickel process)

Electroplating in aqueous solution

(Room temperature operation)

Trivalent cromium conversion coating

Alloy: zinc and nickel distributed uniformly

Zn
- Ni2Zn11(γ)

Zinc's sacrificial protection and anti-rust compound made of Zn-Ni
Dip-spinDip coat paint bake to dry (over 200°C)Organic polymer topcoat or friction modifierA type of paint made of metal flake and silicon-series inorganic binderCoating film works as a barrier layer

 

Dip-spin type coatings, as indicated in Table 3, work as a barrier film to protect the base part or substrate. As with all barrier coatings, performance can be excellent. However, their performance relies heavily on that barrier layer not being damaged or breached. Attempts have been made to improve the coating’s susceptibility to reduced corrosion resistance whenever the barrier layer is damaged, but this issue remains a significant problem, particularly with field or production-scale parts. Torque values for both zinc–nickel alloy plating and dip-spin type finishes are achievable for most desired requirements (Fig. 10).

Torque Tension

The rapid advancement of organic polymer or friction-modifying topcoats enables them to be applied and bonded to either finish. Zinc–nickel alloy plating on a production scale provides a more consistent torque and corrosion-resistant result. This is largely due to the smooth, uniform finish over the entire part, including recessed threaded areas, compared with dip-spin applications, which can suffer from significant thickness variances (Fig. 13). Zinc–nickel electrodeposits are free from the head and thread fill issues of dip-spin coatings.

Galvanic Corrosion

Recently, many applications have demanded required acceptable contact with dissimilar metal components. This is especially true for aluminum substrate parts, which are becoming more popular for automotive components. Due to their differing electro-potential, aluminum parts are attacked with galvanic corrosion. A severe accelerated corrosion test (CCT) comparison between dip-spin and Zn-Ni deposits with and without the presence of a topcoat or electrodeposited coat was recently performed.

The evaluation included corrosion as a function of etching depth and appearance. Galvanic corrosion performance varies depending on the conductivity and characteristics of corrosive production.

From the comparison listed in Table 4, Zn-Ni + tri-chrome conversion + topcoat provides the best performance. A top coat of more than 0.1 µm in thickness is necessary to achieve good galvanic corrosion protection for aluminum components.

Table 4: Galvanic Corrosion Test Summary
FinishCorrosionEtching Depth (µm) 
AluminumBracket
Dip-spinBA50
Zinc + trivalent chromiumCCOver 500
ED coatCB270
Zinc-nickel + trivalent chromiumBA99
Zinc-nickel + trivalent chromium + ED coatBA65
Zinc-nickel + trivalent chromium + topcoatAA0

 

Conclusions

There is a long and proven track record of performance for both alkaline zinc–nickel alloy plating and dip-spin coating technology. Alkaline zinc–nickel coatings are an electrochemical process and, therefore, provide a smooth, more uniform finish when compared to dip-spin coatings (attributes ideal for automotive components). High alloy alkaline zinc-nickel coatings maintain stable performance even after aging, and they can use conventional plating equipment.

Bio

Toshiaki Murai graduated from Miyazaki University (Japan) in 1977 and began his career with Dipsol Chemicals Co., Ltd. In his time with Dipsol, Murai helped develop alkaline Zn, Zn-Ni, acid Zn, neutral Sn-Zn, acid Sn-Pb brightener, and several chromate systems. He has worked for Dipsol of America since 1993.
 

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Comments

Mahmood said

05 January 2012
Interesting studies.
Mahmood

natalia said

11 April 2011
very informative. i just wonder where can i get the bibliography, the references of the figures ? many thanks

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