Corrosion of Common Alloys in Dry Chlorine (Cl2)

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In a couple earlier Engineering Bulletins we showed how Chloride / Chlorine Levels in aqueous environments affect alloy selection and how PREN (Pitting Resistance Equivalent Number) is used to determine pitting corrosion resistance in the face of chloride attack.

Here, we consider another dimension in chlorine piping systems. Dry chlorine is defined as chlorine with its water content dissolved in solution. It could be a liquid or a gas. If the water exceeds its solubility and forms a second aqueous liquid phase, the chlorine is defined as wet chlorine, and becomes extremely corrosive.

The Chlorine Institute’s Pamphlet 6 details how moisture most commonly enters a dry chlorine system.

• Start-up and shutdown
•  Wet pad purge gases
• Exposure to atmosphere

Dry chlorine is not corrosive to steels at ambient temperatures and is commonly shipped and handled in carbon steel equipment, with higher-alloy materials such as Monel 400 and Hastelloy C-276 used for critical parts.

However, temperature and pressure changes can affect the moisture content of the solution, ushering the change from dry to wet chlorine. For this reason, it’s important to monitor these operating conditions.

Corrosion rates of different alloys in dry chlorine and temperature (°C) at which given rates are exceeded are presented in the table below. Keep in mind that moisture will greatly accelerate the attack of any of these materials with the additional danger of SCC (Stress Corrosion Cracking) of stainless steel.

Corrosion of some common alloys in dry Chlorine (Cl₂)

Disclaimer: The info presented here has been compiled from sources believed to be reliable. No guarantee is implied or expressly stated here and the data given is intended as a guide only.

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