Metallographic Etching - The Processes, Reasons to Etch and What Etching Achieves

Topics Covered

Background

Why is Metallographic Etching Used?

Metallographic Etching Processes

Chemical Etching

Electrolytic Etching

Heat Tinting

Background

Metallographic etching is one of a number of processes that takes place during sample preparation for metallographic examination. The steps involved in preparing a sample include:

         Sectioning

         Mounting

         Identification

         Grinding

         Polishing

         Cleaning

         Etching

Why is Metallographic Etching Used?

Metallographic etching is the process of revealing microstructural details that would otherwise not be evident on the as-polished sample. Etching is not always required as some features are visible in the as-polished condition such as porosity, cracks and inclusions.

A properly prepared specimen will reveal properties such as grain size, segregation, and the shape, size, and distribution of the phases and inclusions that are present, while other aspects such as mechanical deformation and thermal treatments may also be able to be determined.

Metallographic Etching Processes

There main etching processes used in metallographic sample preparation are:

         Chemical etching

         Electrolytic etching

         Heat tinting

Chemical Etching

This typically involves immersing the sample in an etchant such or swabbing the surface with an etchant. The etchant selectively corrodes microstructural features. Immersion time or etching time is highly dependent on the system and in most cases requires experience. The selection of the optimum etchant is also very important in sample production.

Deeper etches are preferred for low magnification examinations, while shallow etches are preferred for higher magnification etches.

Electrolytic Etching

Electrolytic etching and electropolishing are in effect the same process, except that electrolytic etching uses lower voltages and current densities. Most electrolytic etching processes use direct current electrolysis. The process uses the specimen as the anode, with the cathode being a highly insoluble, but conductive material. Typical examples are platinum, graphite and stainless steels.

Heat Tinting

Heat tinting, sometimes called thermal etching is the process of oxidizing a sample in a furnace. This induces oxidation of surface features at different rates, to reveal various features.

 

Primary author: AZoM.com

 

Date Added: Jul 20, 2006 | Updated: Jun 11, 2013
Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit