The Handling of
Corrosive Gases
Ca,ses are corrosive a1 ordinary teniperiitures only in the prese,ice of moi~siurp. Re,sistance lo moist gaseous corraske agents can he obtainad in the same imy as resi.s/aitt-e to the same ugeiiterlls when. dissolzred in water, arid uuuikzble materials m d methods for the corriiiioii
cmes arc listed. Direct gaseots atluck a1 high 1ernperulure.s i s 1700 types: surface compound formation, and disirttegration. Some principles can be laid down for resis1ar1,ce to surface attack. .IZetals for resistmce lo high-temnperutare allack are listed from severul sources.
of
:tpproai:hed by a consideration of these different actions of gases on metals: 1. Electrochemicd atback, dependent on the presence of a conduebing film. 2. Direct gaworru attack: ( a ) surface a&&, and (b) d i e integmtion.
HE: meiitioii of currosivc gases brings to thc chemist‘s mind the thought of oxides of nitrogen, hydrochloric a.cid vapors, and sulfurous fumes. A moment’s reflcction, however, will recall to him that these gases are not intrinsically corrosive, for it is well known that liquid sulfur dioxide is safely handled in iron cylinders, and even liquefied nitrogen peroxide has been stored the same way. What then is gaseous corrosion? Or, when are gases corrosive? The present article will attempt to supply some answers to tho second question and to outline some of the methods availal>le ior meeting the challenge of corrosive gases.
TYPES OR GASEOU~ COKKOSION Thcre are a t least two distinct types of action by gases on petals. The first is corrosion in the more commonly accepted sense of the word, which is seldom if eyer e.uhjbited hy single pure gases. This type is exempliiied by the action of moist atmospheres containing corrosive agents, such hydrochloric acid vapor. Here the action is electrochemical iii nature and is, in general, similar to that of the same agent when dissolved in water. The second is a direct combination of the gas with the metal, as the formation of oxide scale on iron when heated in air. A third type may be distinguished from the second in that there is no apparent surface chanpe, yet the structure of the metal is altered, possibly by attack of the minor essential ingredients. The ernbrittlement of steel hy hydrogen at high tempcratnres is an example. The answer to the question, When are gases corrosive?, can be
/. Ihere has been a gruwing appreciation of the necessity fur the presence of a conducting film on the surface of the metal in the first type of corrosion hy gnseous agents. It is not necessary a t this date to review the electrochemical theory of corrosion hy liquids. It should be sufficient to state that all of the prjnciples involved in it are applicable to corrosion by gases, under such conditions that an electrolyte is present on the surface. The only important distinctions arc, that with gas mixtures containing oxygen there is everywhere a plentiful supply of this clement; and the products of corrosion are less likely to he rcniovcd, sincc this would require a higher velocity of a gas than of a liquid reagent. While atmoq~hericcorrosion, the literature on which has reached enormous proportions, will not be corisidercd speeifically here, it ia noteworthy that, the most informative recent work has approached the prohlein from the standpoint of the corrosive gasea present in the atniosplicre and particularly in relation to tbemoisture content. Vernon (%),for example, having previously shown that the corrosion of copper is due to sulfur dioxide, now finds that copper is not attacked appreciably by completely dry air a t ordinary temperatures, even in the presence of as much as 10 per cent sulfur dioxide. In the presence of moisture, however, even small concentrations of sulfur dioxide cause a rapid attack. The rate depends on the humidity, and there is a rapid increase in the rate lrctwcen 63 and 75 per cent relative humidity. He finds
125
126
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further that a t a high humidity tire rate id attack iiicreases with sulfur dioxide content of the air up to ahout 1 pix cent sulfur dioxide, then decreases slightly, and finally rises sharply. This effect is explained by analysis of the corrosion products: those froni atmospheres containing less than 1 per cent sulfur dioxide arc basic; those from stronger mixtures contain an excess of free i;ulfuric acid (no sulfites mere fmmd in any case). The same emphiisis is placed on tlie necessary presence of moistore in the corrosion of power-plant equipment hy flue gases (where solfiir oxides are again t.lie active agent). Johnstone (14) has proved that the dew. point of gases i:ontaiiiing sulfur trioxide and water vapor is eonsiderahly above that of gases eoiitaining water vapor alone, and he has devised an ingrnious dew-point apparatus ( f 9 ) , which is drpendent on tho very same phenomenon as is corrosion-tire formation of an electrically uoodocting film. TIie presence of ferric SUIfate also influences the condensation of moislure, and, to prevent corrosion, tho actual t e n perature of the metal (not oE the gas) must hc above that a t which an acid film condenses. The essential factor, wen in .the rusting of iron, has recently been shown (17) to lie a critical humidity of the atmosphere, dotemined hy t,he gel structure of precipitated colloidal oxiric (rust). This first type of action is, then, iiitimiitr.ly connected with the presence of moisture in thc gases and is dependent on many of the samc considerations &s influence the corrosion in tlie presence of Iiqnids.
The second general type of corrosion hy gases is an effect of entirely different order of magnitude at ordinary t.empcratures, or else appears only a t elevated temperatures. A metal exposed to a gas for which it has any afiiiity will soon becornc covered with R molecular layer of the compound; hut, in order for action to proceed further, the gas must penetrate to lowcr layers. This diffusion is very slow. at ordinary temperatures; consequently this type is often rofcrrcd to as higlitemperat.ure corrosion, thirugh in certain cases the same kind of action is iiotcd at moderate temimraturcs. l'illing aiiri Bedworth (18)enunciated the factors on which surface attack OS metals depends: t,hc dissot:iation pressure of the corrosion product (distinguishing, for example, the noble metals froiri the base); the ratio (molecular weight of compound mnltiplied by density of mietal) divided by (inolecular weight of metal multiplied by density of conipound), wliich determines whetlicr tlic compound Eorms a eontinuous protectire layer over the mirlerlging mctnl; thi: coefficient of thermal expansion of tlie c~oiripumrdcinnparcil >vit,Ii tlrat of tlie nietal; t,lir plasticity, strength, arid adhesion of the cmnliound, and, of course, ita volatility; tlie satoration i:onn!iitratiirn of the gas in tlic compound; and the slmcific diffiisivity of the gas throngh it. If thc cornpound is nm-volatile aiid f(,rins a continuoiis coating orer the metal, tlic rate of at,taek is go.;crned by a welldefined law, since tlic amount of gas diffusing decreases with increasing thinkness of tlie laycr: the extent of the attack increases with the square root of tlic time of exposun?. Tliis parabolic rat.e is characteristic of high-temperature surfaw at,taek, but the smie effect is seeii in certain low-temperatiin,. corrosion results. Vrrnori (82) found that the attack of hydrogen chloride and of liydrogen sulfide on steel f o l l o ~ v ~ ~ ~ l the same parabolic law. In the case of hydrogen sulfide, inoreover, Vernon found tint tlie rate of attack was the samc wlietlrer the specimen was polished or rough, showing that the attack vas uniform over tlie surface and iiot dcpcnilent on local centers of activity, as in tlie case of sulfur dioxide where rough specimens corroded much more rapidly. The disintegration of metals oiidor direct gascoiis attack !t,ype 26) depends 011 quitc a different set of factors, concerning which not so much is knovn a t present. When no protective compound is forined at. the surface, gas a t high temperatures may diffuse into met.al, and, if it there reacts to form eompotinds of different density or otliar properties (ns I)>-
iteci. For resistnnct: to corrosive ga rxmsidered i n get1era.l nliich are reeninto tlic reagent at all dilutims aud all iviiriii
than the stiliitim.;.
This has I m n the I limpliasis is laid
OII
"iriuist,"
warm t r i i d iti tlir 1iri:si:nco for whidi varioii ble Sor rmiderati: tcinpernatret~gthis not required. wi
It lias, thcn,f nieaiis of the exit fan; the can be used. Fused silica wear on tlie hot fan is thereby has been standard for thisapeliminated. p l i c a t i o n ; glass and stoneMorsr I~ALOGEX ACIDVAware, of course, are also used. POILS. Ivf o i s t hydrochloric A newcomer in this field, for mod e r a t e temperatures, is acid vapor in air represents o n e of the m o s t s e v e r e p i p i n g , etc., of a molded 1: o r r 0 s i v e c o n d i t i o n s enphenol-formaldehyde resin. HydroAnoric acid, moist, is cmmtered. Until recently, metals other than platiirunr an evenmore difficult problem or tantalum u-ere not availthan hydrochloric, although, able for this servicc. Hornc when it is substantially analloys are now on the market, hydrous, copper or even iron of composition given by the can be used. I n the rsbsenct, Coirrlesli of Blcrro-Knoz Cn. of oxygen, i'opper can be used table, which show promise in CHHOME-IROY CONDENSER f o r m o i s t vapors. riot a r e s i s t i n g hydrocbloric acid,
INDUSTRIAL AND ENGINEERING CHEMISTRY
130
Vol. 24, KO. 2
TABLE11. TYPICAL COMPOSITION OF ALLOYSRECOMMENDED FOR RESISTANCE TO DIRECT GASEOUSATTACK(I) MAX. TEMP. RECOMMEXDEDFe
c.
%
500 500b 540 595
B’a‘l Bal. Bal.
700 700b 7606 760b
Bal. Bal.
8OOb 800b 8OOb 815 815b 815) 870b 9OOb 9OOb 9OOb 925b 950b 98Ob 98Ob
lOOOb lOOOb 1000
1000 1000 lOOOb
lOlOb 1035b 1095b 1095 1150b 1150b
.
6.5
...
COMPOSITION
C
Cr
Ni
Mo
Si
cu
Mn
w
Sn
co
Ti
AI
%
%
%
%
%
%
%
%
%
%
%
%
...
1.0
.. .. .. ..
... ... ... ...
..
18.0
2.5
0.50
B’a’l. 0.35 0.12
.. 14:o
13.0
..
..
0:io
i:0
0.08
13.5
..
..
Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
0:io 0.62 0.30
ii:s
Bal.
0.12
20.0
...
Bal. Bal. Bal.
6.0
Bal. Bal. Bel. Bal. Bal.
...
0.30
0.15 0.15 0.35
..
0:47
..
.. .. .. .... ..
0.5 0.2 0.2 0.8 0.6
..
17.0 11.0 11.0 18.0 19.0 19.5 15.0 25.0 14.0 20.0 25.0 15.0
ATTACK BY A I R A N D O X I D I Z I X G FUEL GASES‘
29.0 69.0
...
2.0 72.0 99.2 20.0
...
58.0 85.0 60.5
...
18.0 17.0 25.0 19.0 22.0 20.0
.. .. .. 20.0
.. .. .. ..
26.5 36.0 8.5 9.0 7.25 35.0
i:5
26.5
..
...
8.0
..
.. ..
60.0
... ... ...
...
Bal.
...
...
... ...
1.0 0.50
...
..
.. .. ..
58.0
17:o
38.0 25.0 17.5 65.0 23.0
..
..
..
..
70.0 28.0
.. .. .. ..
..
.. 0:75 0.50
Bal. Bal. Bal. Bal.
0.15 0.08
0.10
..
18.2 13.5 17.5 25.0
8.5
... ... ...
650
705 785
Bal. Bal. Bal. Bal. Bal.
0.07 0.20 0.30 0.20 0.40
18.5 25.0 11.5 19.0
2.0
...
...
1:25 0.35 1.50 1.50 0.50
2.5
...
... ... ...
0:55
... ...
.. .. .. ..
1:30
3.40
... ... ... ...
..
...
...
.. .. ..
15.5 4.0
1.12
...
... ... ... ... ... ... ... ...
...
1.35
...
...
...
...
... ... 2.0 2.5 1.5 2.0 1.8
.. .. .. ..
i:50
..
55:O
.. .. .. .. ..
..
..
..
8:O
.. ..
..
...
... ... 1.5
...
...
...
... ... ... ...
8.0
5.0
...
...
216 0.5
... ... ...
0.50 0.50 0.50
...
..
0.88
..
15.0
8.0
..
..
3.25
..
...
..
0.75 0.50
8.5 17.5 60.5 10.0 20.0
...
, . .
... ...
.. ..
...
..
1:25
2.5
..
0175
... ... ... ... ... ... ...
...
A T T A C K BY H Y D R O G E N , N I T R O G E N , A X D A Y M O N I A d
480 540
... ... ...
... 4.0
...
...
...
... ...
...
ATTACK B Y S U L F U R G A S E S
