Descaling – Metallurgical Processes

Topics Covered

Introduction
Types of Descaling
     Mechanical Cleaning
     Aqueous Alkaline Cleaning
     Acid Cleaning (Pickling)
Industrial Applications
References

Introduction

Descaling is the process of removing oxide deposits from a heated stock, either before or during forging operations.

Scales are formed on a metal surface during heat treatment processes. Oxide scales discolor the metal surface and hinder subsequent finishing operations.

Descaling is a metal cleaning process that removes unwanted surface deposits on metals to provide a smooth surface finish and is a part of the pre-finishing processes which include cleaning, stripping and pickling. Of these processes cleaning and pickling are used for scale removal. 

Pre-finishing is vital for subsequent finishing operations like electroplating. Some finishing processes require a high degree of cleanliness, while others require minimum cleanliness.

Physical methods for descaling include use of wire brushes, extra blows, scraping devices, polishing or blasting. Chemical methods that are used for scale removal are acid descaling and alkali descaling. 

Types of Descaling

The various methods of descaling metals are listed below:

Mechanical Cleaning

For metals that react with chemical, the metal is descaled by physical means. Oxide scale deposits can be removed by using wire brushes and scraping devices like wool pads. Abrasive blasting methods and waterjet spraying can also be used for descaling.

Aqueous Alkaline Cleaning

Mild alkaline solutions such as sodium hydroxide, sodium phosphate, sodium metasilicate and sodium carbonate can be used for cleaning the metal surface and removing the oxide scales.

The stronger the concentration of alkali, the faster the cleaning process. This cleaning method is often followed by mechanical cleaning methods.

Acid Cleaning (Pickling)

Acid cleaning, also known as pickling, is yet another process used for removing oxide scale deposits from a metal surface. It is also used to neutralize any remaining alkalis from the previous cleaning process.

This method of cleaning is suitable for ferrous, aluminum and copper alloys. An acid solution, called the pickle liquor, is used for descaling the metal surface. The waste product formed from the pickling of metals is known as pickling sludge.

Industrial Applications

Some of the main application areas of descaling are listed below:

  • Descaling of carbon steels
  • Cleaning of ferrous alloys
  • Oxide layer removal in jewelry.

References


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