Hydrogen (H2) is a co-product of all chlorine manufactured by the electrolysis of aqueous brine solutions. Within a known concentration range, mixtures of chlorine and hydrogen are flammable and potentially explosive. The reaction of chlorine and hydrogen can be initiated by direct sunlight, other sources of ultraviolet light, static electricity, or sharp impact (See CI Pamphlet 121).
Small quantities of nitrogen trichloride (NCl3), an unstable and highly explosive compound, can be produced in the manufacture of chlorine. When liquid chlorine containing nitrogen trichloride is evaporated, the nitrogen trichloride may concentrate to hazardous concentrations in the residue (See CI Pamphlets 21 and 152).
Oils and Grease
Chlorine can react, at times explosively, with a number of organic materials such as oil and grease from sources such as air compressors, valves, pumps, oil-diaphragm instrumentation, pipe thread lubricants. Equipment and piping must be cleaned prior to use to remove any oils. See CI Pamphlet 6 (11.1). Ensure that non-reactive lubricants are used in chlorine service (e.g. Fluorolube® and Krytox®).
Chlorine is neither explosive nor flammable. Chlorine will support combustion under certain conditions. Many materials that burn in oxygen (air) atmospheres will also burn in chlorine atmospheres.
Chlorine has a very strong chemical affinity for many substances. It will react with many inorganic and organic compounds, usually with the evolution of heat. Chlorine reacts with some metals under a variety of conditions (see Section 10.3.3). It is especially important to not use any titanium in dry chlorine service. Chlorine will react with steel and other metals at temperatures above 149°C (300°F). Do not weld piping and other equipment without properly evacuating and purging chlorine from the equipment.
Corrosive Action on Steel
At ambient temperatures, dry chlorine, either liquid or gas, does not corrode steel. Wet chlorine is highly corrosive because it forms hydrochloric and hypochlorous acids. Precautions should be taken to keep chlorine and chlorine equipment dry. Piping, valves, and containers should be closed or capped when not in use to keep out atmospheric moisture such as precipitation or humidity. Materials of construction must be chosen carefully, depending on the conditions that are expected. If water is used on a chlorine leak, the resulting corrosive conditions will make the leak worse.
The volume of liquid chlorine increases with temperature. Precautions should be taken to avoid hydrostatic rupture of piping, vessels, containers, or other equipment filled with liquid chlorine (see Figure 10.4). Any time liquid chlorine can be trapped between two valves, an expansion device should be present.
When a chlorine container is punctured, allowing the chlorine to release, at first, the chlorine will escape rapidly. As the chlorine is released, the container that was under pressure will now attempt to equalize in pressure with the atmosphere. Once the liquid level drains below the puncture point, auto-refrigeration takes place and the rate of release will significantly decrease, although there still is a considerable amount of liquid chlorine remaining in the container. Learn more about this interesting phenomenon in this short, 2-minute video.
Excerpt from Chlorine Basics, Section 2.6. This can be downloaded from our bookstore.