Lesson 3 - Classification and Properties SAQ
This lesson allows you to test yourself on the material covered in the lesson on Classification and Properties and determine the level to which you have met the learning outcomes.
Suggested Response - Q6
**What are the characteristics of the three main zones in a weld? What impact does this have on the potential for corrosion to occur in the area of the weld?
Response:
There are three main zones in a weld; the fusion zone, the unmixed zone and the heat affected zone.
In the fusion zone the metals are completely melted. The fusion zone composition will be a mixture of the base metal and the filler metal. Depending on the cooling the grains that form can be different to that in the base metal.
In the unmixed zone the base metal has melted but not mixed with the filler material.
In the heat affected zone changes in grain structure occur which vary depending on the distance from the weld.
In particular there is a sensitised region where precipitation of chromium carbide occurs, this is based on the temperature experienced.
Welds can contribute to corrosion, as the fusion zone is of a different composition to the base metal, and this can cause galvanic corrosion.
In the heat affected zone the precipitation of chromium carbide can lead to areas that are depleted of chromium, which will then act as an anode and lead to intergranular corrosion.