The key difference between acid zinc and alkaline zinc plating is that the acid zinc plating process has a faster electrodeposition rate whereas the alkaline zinc plating has a lower electrodeposition rate.
Plating process is a preventive measure against rusting. We use zinc plating process often to protect the metal surface or iron and steel against the corrosion caused by rust. Therefore, we apply a thin layer of zinc on the metal surface (substrate) that creates a physical barrier. There are two major forms of zinc plating that we use on different occasions; acid zinc and alkaline zinc plating. The alkaline zinc plating is again in two types as alkaline cyanide plating and alkaline non-cyanide plating.
CONTENTS
1. Overview and Key Difference
2. What is Acid Zinc Plating
3. What is Alkaline Zinc Plating
4. Side by Side Comparison – Acid Zinc vs Alkaline Zinc Plating in Tabular Form
5. Summary
What is Acid Zinc Plating?
Acid zinc plating is the process of electroplating in which we use acid solutions such as zinc sulfate or zinc chloride complexes. Compared to alkaline zinc plating, this is a comparatively new process. Around 50% of zinc plating processes that we use today are acid zinc plating processes. The metal forms that we can electroplating using this process includes cast iron, malleable iron and carbonitrided iron.
Figure 01: A Zinc Plated Coil
The chemical reactions involved in this process are as follows:
ZnCl2 + 2 KCl → K2ZnCl4
K2ZnCl4 → 2K+ + ZnCl4
ZnCl2 → Zn2+ + 2Cl–
There are two major advantages to using this process. Firstly, it has a high cathode efficiency which results in fewer side reactions. And also, it causes faster electroplating rates. Secondly, it requires minimal waste treatment. However, there is an advantage of this process as well; the corrosive nature of the chemical used, resulting in solution laying in recesses, which could be detrimental to the coating if proper rinsing procedures are not followed.
What is Alkaline Zinc Plating?
Alkaline zinc plating is the process of electroplating in which we use alkaline solutions. There is two distinct process of this plating method; alkaline cyanide plating and alkaline non-cyanide plating processes.
Alkaline Cyanide Plating
This was the first available process. The chemical reactions that involve in this process are as follows:
[Zn(CN)4]2- + 2OH– → [Zn(OH)2] + 4CN–
[Zn(OH)2] + e → [Zn(OH)2]–
[Zn(OH)2]- → Zn(OH) + OH–
ZnOH + e → Zn + OH–
However, now this plating process is not in use due to the high efficiency of other processes and the strict environmental regulations. Nevertheless, it has a key advantage. That is; its capability of zinc plating in parts with low current density areas such as tubes.
Figure 02: A Zinc Plating Solution in a Test Cell
Alkaline Non-cyanide Plating
This process has applications in the modern industry due to its reliability, cost-efficient method, etc. The chemical reactions that take place during this process are as follows:
[Zn(OH)4]2- → [Zn(OH)3]– + OH–
[Zn(OH)3]– + e → [Zn(OH)2] + OH–
[Zn(OH)2] → Zn(OH) + OH–
ZnOH + e → Zn + OH–
However, there are few drawbacks in this process. Firstly, the solutions we use in this process has a high content of carbonates. The carbonate formation increases with the agitation and increasing solution temperature. It leads to decreasing the solution conductivity. Hence, it hinders the electroplating process. Compared to this process, acid zinc plating has a conductivity due to the KCl in solution.
What is the Difference Between Acid Zinc and Alkaline Zinc Plating?
Acid zinc plating is the process of electroplating in which we use acid solutions whereas alkaline zinc plating is the process of electroplating in which we use alkaline solutions. There are several differences between acid zinc and alkaline zinc plating. Above all, the key difference between acid zinc and alkaline zinc plating is that the acid zinc plating process has a faster electrodeposition rate whereas the alkaline zinc plating has a lower electrodeposition rate. Another important difference between acid zinc and alkaline zinc plating is that the acid zinc plating process requires minimal waste treatment whereas the alkaline zinc plating processes require considerably high waste treatments.
The below infographic provides more details on the difference between acid zinc and alkaline zinc plating.
Summary – Acid Zinc vs Alkaline Zinc Plating
Zinc electroplating is the most common process that we use to prevent metal surfaces from rusting. There are three forms of zinc plating; acid zinc plating, alkaline cyanide plating, and alkaline non-cyanide plating. The key difference between acid zinc and alkaline zinc plating is that the acid zinc plating process has a faster electrodeposition rate than the alkaline zinc plating.
Reference:
1. “The Zinc Plating Process.” Sharretts Plating Company, 17 Apr. 2018. Available here
2. Sierka, Christopher Eric, “Industrial Zinc Plating Processes” (2015). Theses and Dissert at ions (All). 1283 (pdf)
Image Courtesy:
1.”670657″ by rkit (CC0) via pixabay
2.”Hullcell”By Fstep – Own work, (Public Domain) via Commons Wikimedia