CHAPTER 7 MINERAL ACIDS

SULFURIC ACID The consumption rate of H2SO4 could be used as a yardstick to judge economic conditions. The uses of H2SO4 are production HCl, pickling of steel, fertilizers, dyes, drugs, pigments, synthetic detergents, rayon, petroleum refining rubbers.

7.1 Steel Ordinary carbon steel is widely used for H2SO4 in concentration over 70%. Storage tank, pipelines – 78%, 93%, 98% acids and oleum.

Fig 7.2. Corrosion of steel by H2SO4 as a function of concentration and temperature.

Aeration has little effect. - More dilute acids attack steel very rapidly - Unsuitable above 175F High velocity acid would increase corrosion. Steel pumps not be satisfactory Aeration has little effect.

7.2 Cast Iron Ordinary gray cast iron shows the same picture as steel. - better corrosion resistance in hot strong acid - better corrosion resistance in very hot and very strong acid than stainless high alloys. However, the corrosion rates are high. - not recommended for oleum. Steel is generally preferred over gray iron primarily for safety reasons.

7.3 Chemical Lead used extensively for H2SO4 in the lower concentration ranges.

Fig 7.3 Lead takes over below 70% acid

Fig 7.4 Isocorrosion chart for lead

Rapid attack occurs in concentrated acids because the lead sulfate surface film is soluble. High–velocity acid and solids in suspension can remove the protective lead sulfate coating.

7.4 High–Silicon Cast Iron Cast Iron + 14% silicon best all round corrosion resistance over the range 0 - 100% - Hard, brittle, susceptible to severe thermal shock - not affected by aeration, very resistance to erosion corrosion

- pumps, valves, heat exchanger, pipe and fittings, bubbles caps. - Trade name Duriron - not recommended for fuming acid or for over 100% acid - used at temp as high as 1000F

Fig 7.5 Corrosion of Duriron by sulfuric of concentration and temperature.

7.5 Durimet 20 Fig 7.6 Corrosion of Durimet 20 by sulfuric acid as a function of concentration and temperature.

- used over the entire concentration range - corrosion resistance in oleum - pumps and valves

7.6 Nickel – Molybdenum and Nickel – Molybdenum – Chromium Alloys. Chlorimet 2. 2/3 Ni + 1/3 Mo - good resistance strong acid - poor resistance hot dilute acid

Fig 7.7 Corrosion of Chlorimet 2 and Hastelloy B by sulfuric acid as a function of concentration and temperature

Fig. 7.8 Corrosion of Chlorimet 3 by sulfuric acid as a function of concentration and temperature.

Fig. 7.9 Combined chart for corrosion of six alloys by sulfuric acid.

7.8 Conventional stainless steel - generally not used 7.9 Monel, Nickel, Inconel, Ni – Resist - reducing conditions 7.10 Copper and Its alloys - not used

7.12 Summary Chart Fig. 7.12 Corrosion resistance of materials to sulfuric-corrosion rate less than 20 mpy. Note Factors such as erosion corrosion and contaminants in the acid may change the picture drastically.

7.13 Equipment at Ambient Temperature

Table 7.1 (continue)

7.15 Nonmetallics Fig 7.13 Corrosion resistance of Pfaudler 53 glass to sulfuric acid. (Pfaudler Co.)

Fig 7.14 Corrosion resistance of Durcons 2 and 5 to sulfuric acid.

NITRIC ACID Two general classes - The stainless steels and alloys - The high – silicon irons

The discussion 1. Generally used and suitable for a variety of conditions of temperature and concentration. 2. Used under certain conditions only because of high cost, limited corrosion resistance, resistance only to specific concentrations, or a combination of these 3. Generally not used or not suitable primarily because of insufficient corrosion resistance.

7.16 Stainless steels A large number of stainless steels and alloys, and the choice for a given application will depend largely upon chromium content, fabrication applications and cost.

Average corrosion rate, mpy Table 7.3 Influence of chromium on resistance of low carbon steel to boiling 65% nitric acid % Cr Average corrosion rate, mpy 4.5 8.0 12.0 18.0 25.0 155,000 1,700 120 30 8

7.17 Class I Materials High-silicon iron (14.5% silicon), 18-8 S (type 304), 17% chromium (type 430) High – silicon. – inexpensive, high temp, high conc, erosion

Fig 7-15 Corrosion of quench-annealed 18-8s by nitric acid including elevated temperatures and pressures.

- excellent corrosion resistance at all concentrations  80F - the corrosion resistance  as conc  and temp  - poor resistance to hot very strong acid, fuming nitric acid

An iron-base alloy containing 15-15% chromium - a tank car for shipping - brittleness of casting and brittleness of wrought

Fig 7-16 Corrosion of high-silicon iron by nitric acid as a function of concentration and temperature

isocorrosion – Duriron corrosion resistance  as conc  Durichlor 3% Mo. Not better than Duriron Duriron pumps, valves, heat exchangers, fans, pipe, and small vessel

7.18 Class 2 Materials Titanium < 5 mpy in 65% acid 350F - quite expensive – only material that will do the job - excellent resistance to fumic nitric - not recommended if water content < 1.5% + Nitrogen Oxide > 2.5% Aluminum Excellent resistance to strong acid > 80%

Fig 7-18 Corrosion of aluminum by nitric acid as a function of concentration and temperature

- suitable and used commercially for strong acid + fuming acid  stainless steels are superior in lower conc but aluminum is better in the concentrated Aluminum equipments – cooling coils, condensers, piping, hoods, ducts, storage tank

Table 7.4 Concentration and temperature limits in nitric acid for some nonmetallics Material Temperature, 0F Ambient Elevated Teflon Polyethylene PVC (unplasticized) Butyl rubber Saran Karbate Penton Durcon 100% 60% 50% 10% 30% 70% 100% up to 5000 20% at 1000 40% at 1400 30% at 1500 5% at 1000 10% at 1850 30% at 2500 40% at 1500

7.19 Class 3 Materials Ordinary cast iron, nickel cast irons, magnesium steels and low – alloy steels are rapidly attacked Copper, Ni, Cu-Ni base alloys, brass, bronze, monel, and cupronickels – high corrosion rates.

7.20 Mixed Acids H2SO4 + HNO3 . Fig 7-20 ordinary steel is suitable when the water content is low

Fig 7-20 Corrosion resistance of materials to mixtures of sulfuric and nitric acid at room temperature-less than 20 mpy. (Courtesy G.A. Nelson, Shell Development Co.)

Hydrochloric Acid HCl is the most difficult of the common acid to handle from the standpoints of corrosion and materials of construction. The acid is very corrosive to most of the common metals and alloys.

When aeration or oxidizing agents are also present corrosive conditions may be very rugged. Materials that show very low rated of corrosion are often not economically feasible. Good judgment is required to obtain a good balance between service life and cost of equipment. When contamination is a problem, expensive materials such as tantalum are the only ones that can be utilized.

3. Classifications 1. Generally used and suitable for most applications 2. Used with caution and under specific conditions 3. Generally unsuitable under any conditions and recommended only for trace amounts of acid.

7.21 Class 1 Metals and Alloys Chlorimet 2, Chlorimet 3. Hastelloy B, Hastelloy C, Durichlor, tantalum, zirconium, and Molybdenum. Molybdenum is an important constituent of the alloys.

Fig. 7-21 Corrosion resistance of materials to hydrochloric acid-less than 20 mpy. (Courtesy G.A. Nelson, Shell Development. Co.)

Durichlor - a high-silicon iron containing Mo is much more corrosion resistant to HCl than the alloy without Mo - Used in industry for all concentrations of HCl - Chlorimets and Hastelloys are nickel-base alloys with large Mo contents - Are attacked if aeration or oxidizing ions are present

7.22 Class 2 Metals and Alloys Cu, Bronzes, Cupronickels, Monel, Nickel, Inconel, Ni-Resist, Hastelloy D, Duriron, 316 stainless steel and stainless high alloys Monel is slightly better than nickel and inconel Ni – resist is suitable only for low conc. at room temp.

7.23 Class 3 Metals and alloys Carbon steels & cast irons are never used for HCl Zinc & Magnesium are rapidly attack Tin plate – small amounts of acid Aluminum and its alloys. – oxide surface film are destroyed Lead and its alloys are not recommended

7.24 Aeration and Oxidizing Agents Cu & Cu alloys rapidly attacked under oxidizing conditions.  chlorimet 3 (18% chromium)

7.25 Nonmetallic Materials The nonmetallic have found widespread use – good resistance and immunity to attack by oxidizing ions Rubber–lined steel has been used for many years for vessels and piping for HCl service. Wood finds application as an inexpensive material for dilute acid

Fig 22 Fig. 7-22 Corrosion resistance of Pfaudler 53 glass to hydrochloric acid. (Pfaudler Co.)

7.26 Hydrogen Chloride and Chlorine Titanium is resistant to wet chlorine but not to dry chlorine. Zirconium is resistant to dry chlorine but not to the wet gas

Hydrofluoric Acid HF is unique in its corrosion behavior High – silicon cast irons, stoneware, glass are generally resistant to most acids, but all of these materials are readily attacked by HF. Magnesium resists attack HF and fluorine are toxic

7.27 Aqueous HF Steel is suitable for conc. 60-100% Wrought Monel resists conc. At all temp. Aeration & oxidizing salts increase corrosion of Monel Monel castings are also suitable Silver is used for more severe services such as boiling strong acid

Cu – is suitable for hot and cold dilute sol and for high strengths up to 150F Lead good resistance conc. < 60% at room temp. Stainless – oxidizing salts

Fig 7-23 Corrosion resistance of material to hydrochloric acid-less than 20 mpy. (Courtesy G.A. Nelson, Shell Development Co.)

7.28 Anhydrous HF - The acid is not particularly corrosive 7.29 Fluorine - Dry fluorine gas – is practically non corrosive to metals and alloys. - steel Moist fluorine are extremely corrosive

H2PO4 (Phosphoric Acid) Corrosion depends on the methods of manufacture and the impurities present in the commercial finished product. Fluorides, chlorides, H2SO4 are the main impurities.

7.30 Materials of Construction 316 stainless steel. Little attack Durimet 20 Lead and its alloy. Used to temp  200C conc  80% High – silicon irons, glass, stoneware good resistance to pure acids. Aluminum, cast iron, steel, brass, and the ferritic and martensitic stainless steels exhibit poor corrosion resistance.

Fig. 7-24 Isocorrosion curves, 0 Fig. 7-24 Isocorrosion curves, 0.1 mm/yr (4 mpy) in pure sulfuric acid (solid line) and in sulfuric acid containing 2000 ppm Cl- (dotted lines). 254 SMO = UNS S31254