Question:
We are interested in designing an oven, and there is hydrogen bromide in the area (it is used as a flux). It can sometimes drip onto the oven, and fumes can also enter the unit. We are planning on using 304 stainless steel for the oven design but don’t know if it will hold up. Do you have any information on this or know where to look?

Answer:
Hydrogen bromide (HBr) under standard conditions is a gas, but it can be liquified. The aqueous solution hydrobromic acid is formed when HBr is dissolved in water. Conversely, HBr gas can be liberated from hydrobromic acid solutions by heating or upon the addition of dehydration agents.

A good reference source on the subject is the book A Guide to Corrosion Resistance (Climax Molybdenum, 4th printing). The following data is provided under the section entitled “Corrosive Mediums”:

Compound: Hydrobromic Acid
Concentration: Various
Source: Decomposition products of ethylene dibromide
Industry type: Chemical
Type of Test: F
Average Temperature: 200-212ºF
Test Duration (days): 37
Aeration: None
Agitation: x
Average Corrosion Rate (ipy): Type 304 SS – 0.018cd; Type 316 SS – 0.0035cd

The book provides the following key: F – indicates field or pilot-plant test; x – indicates slight to mild agitation (thus, by inference we are talking about aqueous not gaseous corrosion)

While the corrosion data are given in terms of inches per year (ipy), this does not necessarily indicate that the specimen has corroded uniformly over the surface and at a constant rate. To convert inches per year to inches per month, multiply by 0.0833.

The lower case “c” and “d” after the corrosion-rate figures refer to observed mode of corrosion where the attack was not uniform.

Here’s how to interpret these results. 0.018 inches/year is the total corrosive attack expected, which can be in the form of either pitting corrosion or crevice attack. If the pit corrosion depth is greater than 0.010 inches, staining will also occur.

Another source of information is: P.A. Schweitzer, Corrosion Resistance Tables, Marcel Dekker, New York, (1986).

Here is another useful tidbit: Hydrobromic acid is formed by dissolving gaseous hydrogen bromide in water. It has a pKa of −9, making it a stronger acid than hydrochloric acid. Hydrobromic acid is one of the strongest mineral acids known.