Corrosive Atmosphere

Question:

We have a fairly new facility with the wastewater pretreatment system in a separate room. This wastewater pretreatment system consists of conventional chrome reduction, pH adjustment, flocculation, clarifier and filter press. Despite its newness, primer and coating and general ventilation, the room’s metal structure is already showing signs of severe rust. Any ideas on what is the problem? P.W.

Answer:

The most likely cause for this severe rust is the release of acid vapors into the air and then condensing with water vapor onto the surface of the steel; this condensation is very acidic and will attack steel. The three likely sources of acid vapors is your spent acid holding tank, chrome reduction treatment tank and metabisulfite day tank.

Your spent acid holding tank contains the same hydrochloric acid (aka muriatic acid) as in your acid pickle tanks. Hydrochloric acid has an appreciable vapor pressure (as evidenced by the visible fumes that are emitted from containers of 32% acid when opened) even at the relatively low concentrations of spent acid, thus acid vapors are released from your spent acid holding tank. There are two actions you can take. First, introduce the acid at or near the bottom of the holding tank so as to minimize any turbulence or aeration of the tank that would increase acid vapor emissions. Second, install a cover over the tank and vent to the outside.

At your chrome reduction tank, the sodium metabisulfite reacts with acidic water and can generate some sulfur dioxide (SO2) or sulfurous acid (H2SO3) fumes. Again, these pungent fumes mix with the air moisture and condense out onto the steel. There are several actions for you to take. First, be sure that you are not operating at a pH lower than 2.0. If you find that at times your spent acid drives the pH lower than 2.0, you need to either reduce spent acid feed rate or add a caustic feed to keep pH above 2.0. A pH less than 2.0 does not appreciably increase chrome reduction time but does increase the release of sulfur dioxide fumes.

Second, be sure you are not overdosing the sodium metabisulfite (MBS). To do so, raise your ORP setpoint by 25 millivolt increments and check for hexavalent chromium with your test kit ( if you do not have a test kit, order one today and consistently use it). After running the system at the higher set point for a few hours, check for hexavalent chrome. If hex chrome is not present, raise ORP setpoint by another 25 millivolts and check again. If hex chrome is present, lower ORP setpoint by 25 millivolts and operate at this setting. However, it is good practice to periodically recheck your ORP setpoint since your ORP probe and wastewater constituents can change over time.

Lastly, install a cover over the tank and vent to the outside. Your metabisulfite day tank can be another source of sulfur dioxide vapors as evidenced by its pungent odor. Again, sulfur dioxide mixes with air moisture that then condenses onto steel as an acid solution. May we make three suggestions. First, only provide agitation while you are making up your solution; as soon as you are finished adding the sodium metabisulfite powder, stop the mixer since agitation will increase fume emission. Second, install a cover over the tank with minimal openings since the oxygen in the air decomposes the sodium metabisulfite. Lastly, add a 1/2 pint to a quart (depending upon size of tank) of caustic to help stabilize the solution.

You may also want to evaluate your pH adjustment tank to see if fumes are released from it before neutralization is completed. If so, you should also consider covering and venting tank to the outside.

The amount of exhaust is quite small compared to your existing general exhaust, totaling no more than, say, 500 cu ft per minute (cfm), with duct sizes of 3-6 inches diameter and powered by a small exhaust blower constructed of non-corrosive materials (these acid vapors will destroy a blower constructed of steel in short time). This small amount of exhaust may also save you money, as many shops reduce general ventilation during the winter months because of the cold weather and cost to heat this outside air; winter time is the worst season for the condensation of the fumes and water vapor onto the steel. Combining ways to minimize acid fume generation with targeted exhaust, you should be able to get better control of your structural rust problem.