CHAPTER 7 MINERAL ACIDS. SULFURIC ACID The consumption rate of H 2 SO 4 could be used as a yardstick...
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Transcript of CHAPTER 7 MINERAL ACIDS. SULFURIC ACID The consumption rate of H 2 SO 4 could be used as a yardstick...
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CHAPTER 7
MINERAL ACIDS
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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.
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7.1 Steel
• Ordinary carbon steel is widely
used for H2SO4 in concentration
over 70%. Storage tank, pipelines –
78%, 93%, 98% acids and oleum.
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Fig 7.2. Corrosion of steel by H2SO4 as a function of concentration and temperature.
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- 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.
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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.
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7.3 Chemical Lead
• used extensively for H2SO4 in the
lower concentration ranges.
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Fig 7.3 Lead takes over below 70% acid
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Fig 7.4 Isocorrosion chart for lead
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• 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.
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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
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- 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
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Fig 7.5 Corrosion of Duriron by sulfuric of concentration and temperature.
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7.5 Durimet 20
Fig 7.6 Corrosion of Durimet 20 by sulfuric acid as a function of concentration and
temperature.
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- used over the entire concentration
range
- corrosion resistance in oleum
- pumps and valves
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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
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Fig 7.7 Corrosion of Chlorimet 2 and Hastelloy B by sulfuric acid as a function of
concentration and temperature
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Fig. 7.8 Corrosion of Chlorimet 3 by sulfuric acid as a function of concentration
and temperature.
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Fig. 7.9 Combined chart for corrosion of six alloys by sulfuric acid.
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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
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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.
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7.13 Equipment at Ambient Temperature
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Table 7.1 (continue)
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7.15 Nonmetallics
Fig 7.13 Corrosion resistance of Pfaudler 53 glass to sulfuric acid. (Pfaudler Co.)
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Fig 7.14 Corrosion resistance of Durcons 2 and 5 to sulfuric acid.
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NITRIC ACID
Two general classes
- The stainless steels and alloys
- The high – silicon irons
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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.
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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.
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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
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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
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Fig 7-15 Corrosion of quench-annealed 18-8s by nitric
acid including elevated temperatures and pressures.
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- excellent corrosion resistance at all
concentrations 80F
- the corrosion resistance as conc
and temp
- poor resistance to hot very strong
acid, fuming nitric acid
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An iron-base alloy containing 15-15%
chromium
- a tank car for shipping
- brittleness of casting and brittleness
of wrought
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Fig 7-16 Corrosion of high-silicon iron by nitric acid
as a function of concentration and temperature
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• isocorrosion – Duriron
• corrosion resistance as conc
• Durichlor 3% Mo. Not better than
Duriron
• Duriron pumps, valves, heat
exchangers, fans, pipe, and small
vessel
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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%
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Fig 7-18 Corrosion of aluminum by nitric acid as a function of concentration and temperature
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- 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
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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
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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.
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7.20 Mixed Acids
• H2SO4 + HNO3 . Fig 7-20
• ordinary steel is suitable when the water
content is low
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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.)
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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.
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• 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.
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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.
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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.
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Fig. 7-21 Corrosion resistance of materials to hydrochloric acid-less than 20 mpy. (Courtesy G.A.
Nelson, Shell Development. Co.)
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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
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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.
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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
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7.24 Aeration and Oxidizing Agents
Cu & Cu alloys rapidly attacked
under oxidizing conditions.
chlorimet 3 (18% chromium)
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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
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Fig 22
Fig. 7-22 Corrosion resistance of Pfaudler 53 glass to hydrochloric acid. (Pfaudler Co.)
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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
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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
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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
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• 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
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Fig 7-23 Corrosion resistance of material to hydrochloric acid-less than 20 mpy. (Courtesy G.A. Nelson, Shell Development Co.)
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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
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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.
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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.
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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