The key difference between pickling and passivation is that the pickling is the process that we use to remove impurities on a metal surface whereas the passivation is the protection of a metal surface against corrosion.
Both pickling and passivation are processes that we can use to protect a metal surface. We use the term pickling regarding food as well. However, here, pickling is a form of metal surface treatment. Here, we clean the metal surface. Passivation, on the other hand, is making a material “passive” to corrosion. Unlike in pickling, here we protect the metal surface before it gets any impurity.
CONTENTS
1. Overview and Key Difference
2. What is Pickling
3. What is Passivation
4. Side by Side Comparison – Pickling vs Passivation in Tabular Form
5. Summary
What is Pickling?
Pickling is the process of treating metal surfaces in order to remove any impurities on the surface. The impurities may include stain, rust, scale, inorganic contaminants, etc. The metal on which we use this process are iron and its alloys, copper, precious metals such as silver, aluminium alloys, etc.
The chemical agent that we use in this process is “pickle liquor”. It usually contains acids; strong acids such as HCl and sulfuric acid are common. It contains some other ingredients as well. For example, wetting agents, corrosion inhibitors, etc. This pickling process is common in cleaning steel surfaces in steelmaking processes. The need for this process is to remove substances on the metal surface which may affect the further processing of the metal such as plating and painting. Descaling is an important step of this process. Many hot working processes leave a discolouring oxide layer (scale) on the surface of the metal. We can remove this scale layer via dipping into a vat of pickle liquor.
However, there are some disadvantages to this technique. Among all, this process is difficult to handle since the pickle liquor is corrosive (it contains strong acids). Moreover, hydrogen embrittlement is another problem for some alloys. As another disadvantage, it produces pickle sludge as a waste product of this process. Ex: spent pickle liquor is a hazardous waste.
What is Passivation?
Passivation is the process of making a material “passive” to corrosion. In other words, it protects a metal surface from corrosion. After the passivation of a metal, the metal becomes less affected by the environment. In this technique, we form an outer layer as a shield material. We can apply it as a microcoating. We can apply this coating via a chemical reaction or as a spontaneous reaction (we can keep the metal in the air for oxidation). Moreover, the passivation of metal occurs only under certain conditions. It is very useful to preserve the appearance of the metal.
Figure 01: Tarnished Silver
The metal that undergoes this process includes aluminium, ferrous materials, stainless steel and nickel. Most of the metals form an oxide layer when we expose it to normal air, i.e. tarnishing the silver surface. But in some metals such as in iron, rust formation occurs in open air. This can reduce the amount of metal. The corrosion coating is important here because it reduces the further corrosion.
What is the Difference Between Pickling and Passivation?
Pickling is the process of treating metal surfaces in order to remove any impurities on the surface. We can do this using pickle liquor. Thus, it protects a metal surface against impurities on the metal surface. Passivation is the process of making a material “passive” to corrosion. This is the key difference between pickling and passivation. Moreover, passivation is sometimes spontaneous and natural (ex” formation of an oxide layer in open air), or we can do this via a chemical reaction. In addition, it protects the metal surface even before it is exposed to the normal air.
Summary – Pickling vs Passivation
Metals are most of the times, very reactive when we expose it to normal air. Pickling and passivation are two techniques we can use to protect the metal surface. The difference between pickling and passivation is that the pickling is the process that we use to remove impurities on a metal surface whereas the passivation is the protection of a metal surface against corrosion.
Reference:
1. “Pickling (Metal).” Wikipedia, Wikimedia Foundation, 3 July 2018. Available here
2. “Passivation (Chemistry).” Wikipedia, Wikimedia Foundation, 27 July 2018. Available here
Image Courtesy:
1.’72414′ by JamesDeMers (CC0) via pixabay