Saturday, May 19, 2012

The Hazards of Acid and Caustic Baths

Acid baths are widely used in electroplating. The most common of these acids are hydrochloric, nitric, nitric-hydrofluoric and sulfuric acids. Also, in what is called a “bright dip,” nitric and sulfuric acids are combined to give a shiny, mirror-like surface to metals and alloys such as cadmium, copper, nickel and silver.

 Direct contact with these acids can result in:

  • severe skin burns 
  • repeated skin contact will cause scarring and open, ulcerated wounds which are slow to heal 
  • burns on the eye can result in impaired vision and even blindness Among the acids, concentrated sulfuric acid causes unusually severe burns and eye damage. 

Hydrofluoric acid in contact with skin destroys tissue. It penetrates deep where it can destroy soft tissues and bone, and cause electrolyte imbalance (particularly with calcium). These acids can release vapors, gases and mists which can cause serious damage to the eyes, nose, throat and lungs. The extent of vapors and mists depends on the temperature of the bath and air circulation in the room. Vapors and mists released by acid baths can dissolve in the moist tissue of the eyes, nose, throat and lungs, and cause irritation and burns of the tissues.

At relatively low vapor levels, the vapors dissolve before they get deep into the lungs, and the irritation is felt in the upper respiratory system, the nose and throat, and on the eyes. If such exposures persist, the irritation and burning give rise to nosebleeds and sinus problems in the case of hydrogen fluoride exposure (can be a by-product of fluoroboric acid which is used in some plating baths).

 Hydrogen fluoride vapors also cause digestive symptoms, including nausea, vomiting, abdominal cramps and diarrhea. Also, chronic (long-term) exposure can discolor, damage and even dissolve the surface enamel of the teeth. In particular, hydrogen chloride, fluorine and nitric acid vapors can discolor the teeth, and both sulfuric and hydrochloric vapors can also cause erosion of the enamel in exposed teeth. In case of sudden, extreme over-exposure, these vapors can reach deep into the lungs and cause severe lung reactions and even death, usually from pulmonary edema (build-up of fluid in the lungs).

A number of effective control measures can be taken to reduce the dangers of acid baths.
These include:

  • good local exhaust ventilation to remove mists, vapors and gases above the tanks; experience has shown that bright-dip baths of nitric and sulfuric acids need closer fitting hoods and more effective exhaust than other types of acid baths; the rinse baths may need ventilation as well; 
  • mist reducing agents and foam blankets on the surface can reduce misting; 
  • baths should be covered when not in use; 
  • chemicals should be added to the baths carefully to reduce splashing; 
  • full protective clothing should be provided and worn at all times, including full face shield, chemical-type goggles, rubber gloves, boots and aprons; 
  • eating, drinking and smoking near tanks should be prohibited; 
  • emergency wash facilities, including shower and eye wash station should be located nearby. 

Acid and caustic baths complement each other. Acid baths are best for removing scale, rust and oxide coatings from metals, caustic (or alkaline) baths for removing oil, soils, buffing compounds and paints. The most common caustics are sodium hydroxide and potassium hydroxide (also called caustic soda and caustic potash, respectively). Most caustic baths also contain a variety of special purpose additives. Many baths use electrodes and pass electric current through the fluid to improve the cleaning effects.

These electrodes also release gas bubbles, which increases the mist above the tank. Caustics in concentrated solutions are even more corrosive to the skin and eyes than acids. Direct contact with caustics can result in:

  • deep, painful burns to the skin 
  • destroying the eye tissue if splashed into the eyes Like acids, caustic vapors, mists and sprays irritate the eyes, nose, throat and lungs. 

Persistent vapor exposure can cause a hole to form in the cartilage between the nostrils of the nose (called a “perforated septum”). Lower mist levels can cause dermatitis. Also, like acids, very high vapor exposures can cause pulmonary edema and death.

The control measures for acid baths apply to caustic baths as well. However, because of the rapid action of caustics on the eye, the need for eye goggles which fully cover the eye and protect it from splashes in all directions is very important.

A face mask is no substitute for these goggles unless the mask encloses the face or head. Also, emergency shower and eyewash fountains must be located nearby the caustic bath. In case of a splash, the eye should immediately be washed for a full 15 minutes.