Corrosion Metah and alloys generally used and suitable for handling hydrochloric acid are discussed bg Mar8 6. Fontanu of metals and alloys for hydrochloric acid service is divided into three classifications as follows: (1) generally used and suitable for most applications; (2) used with caution and under specific conditions-for example, concentrations of less than 5%; and (3) generally unsuitable under any conditions and recommended only perhaps for trace amounts of acid. Classes 2 and 3 were discussed in the August column. The metals and alloys in class 1 are as follows: Chlorimet 2, Chlorimet 3, Durichlor, Hastelloy B, Hastelloy C, Hastelloy A, and tantalum. The compositions of these materials are shown in Table I. Molybdenum is an important constituent in all of the alloys listed. Tantalum is an expensive material but is often used where contamination is to be avoided-for example, steam tubes for heating C.P. acid. This material has high strength and thin-walled tubes (0.002 inch) can withstand substantial -steam pressures. Tantalum shows excellent resistance to hydrochloric acid.
D
ISCUSSION
TABLEI. NOMINAL ~OhfPOSITIONS OF ALLOYSFOR HYDROCHLORIC ACIDSERVICES Mo Duriohlor Chlorimet 2 Chlorimet 3 Hoatelloy A H ~ t e l l o yB H ~ t e l l o yCa Tantalum Also contains
3 32 18 21 28 17
3.75
Per Cent Si Cr C Fe 16.5 Nil 0.85 Balance 1 max. Nil 0.10 3 max. 1 max. 18 0.07 max. 3 max. 1 max. 1 max. 0 . 1 2 max. 19 1 max. 1 ma%. 0 . 1 2 max. 5.5 1 max. 1 6 . 5 0 . 15 max. 5.5 (Commercially pure metal) to 4.75% wolfram.
Ni Bai&x Balance Balance Balance Balance
Dnriehlsr
Durichlor is a high silicon cast iron containing molybdenum. The addition of this element greatly improves the resistance of high silicon iron (Duriron) to hydro-
chloric sacid. Durichlor shows marked passivity effects in that the initial rates of corrosion are fairly high but corrosion decreases rapidly with time. The time factor here is measured in hours so the effect is quite rapid. Durichlor is a relatively inexpensive material and is the most widely used on a tonnage basis, of the materials listed in Table I. It is used in industry over the entire concentration range of hydrochloric and muriatic acids a t temperatures below 200’ F. Strong boiling acid attacks this material. I t is available in only the cast form and is not readily machined except by grinding. I t is hard and erosion resistant but has poor mechanical properties as compared to the other alloys listed. Substantial concentrations of ferric chloride in strong acids tend to pit Durichlor. In weaker acid such as 5%, ferric chloride is not particularly bothersome. Iron usually found in muriatic acids and many byproduct acids is not often detrimental in once-through systems. Closed recirculating systems where the iron content builds up with time may result in pitting of Durichlor. Aeration, or lack of it, does not appreciably affect the corrosion resistance of this alloy. In connection with case histories or actual applications, Durichlor pumps are giving very satisfhctory service handling 30% hydrochloric acid and also sludges containing 10% acid. A life of a t least 4 years was obtained in pumping 35% acid a t 140’ F. Durichlor self-priming pumps show up to 8 years’ life in a galvanizing plant handling 5% hydrochloric acid containing “strong concentrations” of ferrous and ferric chloride a t 175’ F. (Continued on page 118 A )
L
L.
0
f
B a
a f
I
I
e
G
c
L
t
L
0 c I
1: 5
8 Percent
Boiling
Fiwn 1
HCI
Pwcrnt HGI Figure 2 111A
Pwcrnt HCI in IO%
Figure 3
hCI3
Corrosion Nickel baee a h g 8
Aside from Durichlor and tantalum all of the iiiczterials listed in Table I are nickel-base alloys. These alloys are quite expensive but find extensive use for handling this severe corrosive. Chlorimet 2 and Hastelloy B are the most resistant of these alloys. They show good resistance to boiling acid over the entire concentration range as shown by Figure 1 and Table 11. However, these two alloys are subject to greatly increased attack if positive aeration, ferric chloride, cupric chloride, or other oxidizing agents are present. An example of the destructive action of air-sparged acid on these alloys was described in the August column.
TABLE11. CORROSION OF HASTELLOYS B Y HYDROCHLORIC -4ClD" Corrosion Rate. Mils/Year HCI. % Teing., O E'. A B C1386 2 io 33 1i o 11 2 Hoiline 158 130 32 6 Boiling 12 370 5 158 85 33 340 10 260 12 10 Boiling 2600 150 43 228 15 158 Boiling 1000 14 1600 15 110 35 880 20 158 950 23 4303 Boiling 20 10.5 158 41 18 25 1080 19 35 37 158 a D a t a from Haynes Stellite Bulletin, "Hastelloy." b All tests a t 158' F.aerated.
Chlorimet 3 contains 18% chromium and accordii gly shows good resistance to oxidizing environments. Unfortunately, it is rapidly attacked by strong acid at moderate temperatures as shown by Figure 2. Figure 3 is cz plot of data on corrosion of Chlorimet 3 by hydrochloric acid containing 10% ferric chloride at room temperature. Good corrosion resistance is exhibited up to about 15% concentration. In strong acid the alloy is destructively attacked. Figures 1, 2, and 3 are from a paper by W. -4.Luce, "Cast Nickel- Molybdenum and Nickel-Molybdenum-Chromium Alloys for Severe Corrosion Services," Chena. Eng. Progress, 44,453-8 (1948). Data on Hastelloy C, which is also a chromium-bearing alloy, and Hastelloy A, are given in Table 11. The latter is a nickel-molybdenum-iron alloy which finds application in hydrochloric acid services a t room temperature or slightly higher. It is susceptible to rapid attack by ferric and cupric salts and also by aeration ill hydrochloric acid. Chlorimet 2 and 3 are available in only the cast form and are produced by the Duriron Company. The Hastelloys are available in both the cast and wrought forms and are made by the Haynes Stellite Company.
112 A
