This article page is from a segment of the Metalsmith Magazine (1986 Winter), “Health Hazards”, discussing questions from readers and answers provided by Linda Weiss-Edwards herself.

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Dear Linda,

Here is a copy of a paper on hydrofluoric acid, researched by my associate Stephanie Lyall. The contents of the paper were examined by the Arizona Poison Control Center and approved. Folks keep showing up with hydrofluoric acid in their studios with no idea of how dangerous it is, and we felt the need to at least put all the information in one place.

– Bill Seeley, Reactive Metals Studio

Dear Bill,

Thank you for your hydrofluoric acid paper I think the information would be valuable and appreciated by the Metalsmith readership.

Hydrofluoric Acid: Use and Safety

General

Hydrofluoric acid (HF) is a clear, colorless liquid having the unique property of attacking glass and any silicabased material. It is also one of very few substances that will attack titanium, niobium, tantalum, zirconium and hafnium. Although classified as only a moderately strong acid, HF is extremely dangerous, capable of causing severe burns and poisoning if handled carelessly.

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Before actually bringing HF into the shop, be sure to have a complete plan, from storage of the acid and solutions, through use and disposal of waste, to emergency safety procedures. The plan should include a call to the doctor to be sure an antidote is available. as well as to the fire department, to notify them of hazardous material storage.

HF is commercially available from chemical supply houses in concentrations to 70%, 48-50% being generally most available. Technical grade HF is adequate for all metal and glass finishing purposes. Due to its hazardous nature, HF is sometimes difficult to obtain since suppliers worry about liability.

Dangers

HF is an extremely hazardous substance! Exposure to any form of HF can be hazardous. The fumes and liquid, even in dilute concentrations, can cause burns to the eyes, skin, respiratory and digestive tracts. Burns caused by dilute solutions (as low as ½% HF) are especially insidious, as they might not be felt until hours after the exposure. In extreme cases, surgical removal of tissue can be required, and even moderate cases of penetration of acid beneath the skin can require injections of calcium gluconate.

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HF fumes at concentrations greater than 48%. Damage occurs when the HF molecule contacts moist mucous membranes, producing hydrofluoric acid. The fumes are extremely acrid, unpleasant and detectable at very low concentrations.

First Aid

Emergency information is available by calling your POISON CONTROL phone number found inside the front cover of most phone books. In the event of skin exposure to concentrated acid, flush the area with copious amounts of water, while simultaneously removing contaminated clothing. Continue the water flush for at least 15 minutes, then apply an ice-cold saturated solution of epsom salts* for at least 30 minutes. Always call a physician if you are exposed to concentrated acid, or if you feel a delayed burning sensation. If exposed to a weak solution, wash immediately with soap and copious amounts of water. If HF is gotten into the eyes, flush them with water for at least 15 minutes and call a physician immediately. If inhaled, immediately get to fresh air. If swallowed, do not induce vomiting, but give plenty of fresh water, and call the physician immediately. Harshaw Chemical, a manufacturer of HF, recommends calling a physician for any exposure.

Storage

HF should be stored in a well-ventilated area in labeled polypropylene or other acid storage containers (never glass). Food containers should never be used for storage of solutions or as etching tanks, unless they are clearly and permanently marked poison. All HF handling areas should be kept scrupulously clean, and all pets and people not briefed on the hazards of HF should be excluded from the area.

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Personal Safety

Chemical goggles, neoprene or other HF rated gloves and apron are the minimum personal safety equipment needed for handling HF solutions. If concentrations greater than 30% are used, a full face shield and complete protective clothing should be worn.

Always use chemistry lab procedures when handling acids. The area should be well organized, uncluttered, with neutralizer readily accessible. Never sniff chemicals or experiment by adding other chemicals to the solutions. Deadly combinations could result.

Spills

Treat all spills and wet spots in the vicinity of acid use as though they were acid. Take no chances! Spills should be neutralized with excess hydrated lime (Ca(OH2). Lime is available in 50 lb. bags at lumber yards. This is a reasonable amount to have on hand. Sprinkle the lime onto the spill until it has the consistency of heavy paste. Scrape up the mess, then use baking soda in water to check for complete neutralization. Lack of effervescence indicates that the acid has been completely neutralized. Neutralization occurs when the calcium (CA+2) in lime combines with the fluorine ion (F) to form calcium fluoride (CaF2, the composition of the mineral fluorite), which is stable. The H+ of the acid combines with the OH of the lime to yield water.

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Baking soda (sodium bicarbonate, NaHCO3) should not be used initially to neutralize spills because although it neutralizes the acid, it also produces sodium fluoride (NaF)**, which is toxic. Baking soda also bubbles vigorously (hydrogen gas production), which can spread the mess.

Solution Preparation and Use

When making up acid solutions, always add acid to water slowly and carefully to minimize spattering. Polyethylene or polypropylene graduated cylinders should be used for HF measurement. Etching tanks should also be polyethylene or polypropylene. Always mix and use solutions with adequate ventilation. A chemical hood is ideal, but etching can be done outside or with positive ventilation. All acid handling equipment should be periodically inspected. If checks or brittleness occur, the equipment should be replaced.

Keep solutions covered and labeled when not in use. HF is very corrosive to glass and most metals and should not come into contact with these materials. HF should not be used in an enclosed space, since the reaction produces hydrogen gas, which can be explosive. Never allow other chemicals, including household cleansers, to contact the solutions.

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If the solution activity is too vigorous, dilute it with water. If too slow, add measured amounts of acid to the solution. With use, the fluoride in the solution will become depleted, making the addition of HF or a fluoride salt necessary. Keeping accurate records of conditions and modifications to solutions will help to make reproduction of results possible. When undesirable or uncontrollable effects begin to occur, dispose of the solution and make up a new one.

Always wash with soap and water after using FH, and wash all contaminated clothing before reuse. Also, neutralize all containers and tools, including safety gear, with a lime and water slurry.

Disposal

Check local, state, and EPA regulations before disposing of metal or glass finishing solutions. A small quantity of waste HF solution, up to several gallons, can usually be disposed of by flushing it down the drain with copious quantities of cold water, alternated with, or followed by, a lime slurry, to minimize damage to plumbing. It can also be neutralized with lime in a trench outside and then covered.

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Metal Finishing

The parts to be treated are cut, formed, de-burred, thoroughly degreased, and masked with a suitable resist (if patterns are desired). A perforated polypropylene or other acid resistant parts basket is used for dipping. Agitation of the parts is beneficial to ensure an even finish. Plastic tongs, tweezers and titanium wire can be used to hold single pieces. The parts are quickly rinsed in running water and dried after the desired finish is obtained. A more even finish will result with several short dips followed by rinses than by a single long soak. Lime slurry poured down the drain before and after rinsing will minimize damage to plumbing.

If dark stains occur on the metal after the initial dip, they can usually be removed by a second dip in a 50% water, 50% nitric acid solution.

The approximate formula of the etchant/brightener for titanium used in industry is 35% nitric acid (HNO3). 3% HF, the remainder chlorine-free water (chlorine may cause staining) by volume. As a rule of thumb, titanium will etch and brighten in weaker solutions than niobium, and niobium in weaker solutions than tantalum. Also, a more matte finish is seen in HF/nitric acid solutions, while in general, a brighter finish results from HF/nitric acid solutions with other mixed acids. Never, ever, dip the reactive metals into concentrated HF, or any other concentrated acid! An explosion could result.

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Specific Formulas

For titanium, zirconium and hafnium:

90 ml nitric acid (HNO3) (conc.)
10 ml hydrofluoric acid (HF) (48-50%)
1000 ml water

or

30 ml nitric acid (HNO3)
90 ml hydrochloric acid (HC1) (conc.)
20 ml HF (48-50%)
50 ml water

For niobium and tantalum:

25 ml nitric acid (HNO3) (conc.)
50 ml HF (48-50%)
500ml water

or

300 ml nitric acid (HNO3) (conc.)
300 ml sulfuric acid (H2SO4)
100 ml HF
300 ml water

Rinse, then dip again in:

350 ml nitric acid (HNO3)
200 ml sulfuric acid (H2SO4)
350 ml water

Materials checklist

  • Acid-resistant gloves
  • Chemical goggles/face shield
  • Acid-resistant apron
  • Baking soda
  • Epsom salts
  • Lime
  • Acid-resistant measuring equipment
  • Acid-resistant containers
  • Acid-resistant tongs or tweezers
  • Baking soda (sodium bicarbonate)

* To make a saturated solution of epsom salts, mix as much of the salts as will dissolve in a volume of room temperature water; then chill and add ice.

** Sodium fluoride (NaF) is used in extremely dilute solutions to fluoridate municipal water supplies and toothpastes.

The preceding information was compiled from several sources as well as from personal experience. While not exhaustive, it is correct to the best of our knowledge. We assume no responsibility and hereby disclaim all liability for any injuries, and/or damages, and/or loss of profit resulting from the use or attempted use of the chemicals, formulas and techniques described herein.

More comprehensive technical and medical data can be found in the “Chemical Safety Data Sheet SD-25” on HF published by the Manufacturing Chemists Association, 1825 Connecticut Ave., NW, Washington, DC 20009, and the “Material Safety Data Sheet” on HF from the Harshaw Chemical Co., 1945 East 97th St., Cleveland, OH 44106.

An inorganic chemistry lab manual is an invaluable reference source for chemical handling information. If there are any doubts as to the proper use of any chemicals, an expert should be consulted. Often, a local university is an excellent source of information.