CHAPTER 7 MINERAL ACIDS. SULFURIC ACID The consumption rate of H 2 SO 4 could be used as a yardstick to judge economic conditions. The uses of H 2 SO.

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.

- More dilute acids attack steel very

rapidly

- Unsuitable above 175F

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 1000F

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

Note Factors such as erosion corrosion and contaminants in the

acid may change the picture drastically.

Fig. 7.12 Corrosion resistance of materials

to sulfuric-corrosion rate less than 20 mpy.

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 discussion1. 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.

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 80F

- 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 350F

- 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

MaterialTemperature, 0F

Ambient Elevated

Teflon

Polyethylene

PVC (unplasticized)

Butyl rubber

Saran

Karbate

Penton

Durcon

100%

60%

50%

50%

10%

30%

70%

60%

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 150F

• 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 200C

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, 0.1 mm/yr (4 mpy) in pure sulfuric acid (solid line) and in sulfuric acid containing 2000

ppm Cl- (dotted lines). 254 SMO = UNS S31254