"Steel Corrosion in Chlorine Storage Rooms"

Question:

"My question is regarding appropriate materials to use in a room storing compressed chlorine gas containers. The chlorine gas does not leak from the containers during storage, but in tank changeout, some gas will leak. I know that steel is used with dry chlorine gas, but not with "moist" chlorine gas. I believe this is due to the formation of hydrochloric acid when the elemental chlorine reacts with the water. The hydrochloric acid then corrodes the steel. My question is: Is the corrosion due solely to the presence of chlorides from the hydrochloric acid or does the hydrogen ion play a part as well? If it is just chlorides, then can I use materials for such things as fans and louvers that are suitable in marine environments? How can the aluminized steel be applied in heat exchangers? What kind of base metal can be aluminum coated and also be applied in heat exchangers?"

Answer:

Dry chlorine gas, when contacted with moisture, will form a hydrochloric acid (HCL) solution. HCL is one of the more corrosive acids due to its strong reducing nature. Corrosion due to the presence of chlorides can be more or less aggressive, depending on whether the aqueous media containing chlorides is neutral or acidic. For example, seawater contains approximately 3% salt, as sodium, calcium and magnesium chlorides. The pH of seawater will normally be neutral to slightly alkaline, depending on other factors such as dissolved carbon dioxide, algae, etc. Hydrolyzed chlorine gas (chlorine gas in water) will be acidic. As a general rule, with equal amounts of chlorides, acidic solutions will be more corrosive than neutral chloride solutions, based on hydrogen evolution as an additional cathodic depolarizer, in addition to oxygen. It is difficult to answer your specific question regarding the use of materials "suitable" for use in marine environments since that list can be diverse and to some extent the answer depends on the total exposure time and intended function and stress level of the material. If you reply to this post with specific alloys and intended use I might be able to clarify your question. Most of the common alloys that would be used as heat exchanger tubing can be aluminized, including carbon steel, low alloys steels and stainless steels. The diffused aluminum layer in the higher alloyed materials may be less than in carbon steel due to reduced diffusivity and solubility. I'm not certain whether the copper alloys can be aluminized. The more expensive, corrosion resistant alloys are not normally aluminized in aqueous service, except for specific corrosive environments, due to their inherent corrosion resistance. Austenitic stainless steels have been aluminized in high temperature sulfur and coking environments due to aluminum's excellent resistance to those environments.

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