Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Isocorrosion Diagram

Last updated: August 1, 2017

What Does Isocorrosion Diagram Mean?

An isocorrosion diagram is a kind of tool used to recognize high-corrosion situations during the design process of equipment. It makes use of lines that form curves to indicate corrosion rates at a particular point within a reactor. A detailed and precise understanding of how an isocorrosion diagram works can serve as a guideline early in the design phase.

The diagram consists of lines, each representing an alloy and its corrosion rate at 0.1 mm a year. At temperatures or concentrations above the line, the corrosion rate tends to be elevated. If the concentration is below the line, the speed of corrosion is less.


Corrosionpedia Explains Isocorrosion Diagram

A useful application of the isocorrosion diagram is detecting the speed and probability of corrosion in certain materials. For instance, an alloy 28 (a nickel alloy) reactor was pied in about two months by sulfuric acid that ran down its inside wall. Using an isocorrosion diagram to evaluate this condition in detail would reveal that this is likely to happen. Thus, this unpleasant situation could be avoided.

In the case mentioned above, sulfuric acid is at 20°C (68°F) and 80% concentration at the point of injection. It must be noted that detrimental contaminants are not present. If you examine the corrosion data of the above-mentioned conditions, minimal metal loss can be observed. But then, when the acid reaches high temperatures, is aerated, dissolves or becomes tainted by chloride, the speed of corrosion accelerates considerably.

The abrupt alterations in terms of corrosion resistance involving metals are easily viewed using an isocorrosion curve or diagram. Typically, the data used in the diagram are created through lab tests and investigation of real-world conditions. Essentially, the curves on the chart reflect the situations wherein the degree of metal degradation is similar. Using several curves in the isocorrosion diagram will show a better appreciation of the effects related to a change of condition. Some of these conditions include concentration, temperature and contaminants.

At times, it is required to superimpose data from various sources into a single chart in order to come up with a group of curves. Corrosion rates can drop or rise significantly, even with the slightest change in conditions.


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