Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
37
Reactions during Corrosion of Metals in Organic Solvents E. Heitz' and C. Kyriazis Dechema-lnstitut, D-6000 FrankfuN Main, Federal Republic of Germany
Typical for corrosion of metals in organic solvents is the great variety of systems metaVmedium which is a consequence of the huge number of organic compounds. However, the vast field can be made surveyable by application of three simple classification principles. These are based on the physicochemical properties of the aggressive medium. Likewise, these properties determine the type of corrosion reactions involved in the metal dissolution process.
Introduction With the development of new technologies, especially in industrial organic chemistry and in petrochemistry, fuel technology and battery technique materials problems are arising which in many cases cannot be solved by applying corrosion protection principles as they are known from corrosion in aqueous solvents. Features different from aqueous corrosion have to be considered and will be shown in this paper. Since coatings are organic compounds with specific groups which are related to the solvent molecules of the type discussed here, conclusions drawn can be related to coatings systems. Considering corrosion systems composed of a t least a material phase, a medium phase, and a phase boundary, the following elaboration will be confined to the phase boundary and the medium phase since they are of predominant interest in corrosion in organic solvents. Phenomenology From many corrosion failures previously found in industry by the use f common construction metals and alloys in organic solvent environment, many examples are known in the literature (Heitz, 1974). These case histories show that the phenomenology of corrosion damages in organic solvents does not exhibit new corrosion types. Well known is the uniform corrosion of steel in pure organic acids and alcohols. A case of pitting corrosion may be seen in Figures 1and 2, in which a T i and Mo stabilized stainless steel of type CrNiMo 18-10-2 (AISI 316) has been attacked by perchloroethylene in the mechanical seal region. Common also is the case of pitting of A1 in a number of organic solvents such as chloride contaminated alcohols and other protic solvents. Uniform and erosion corrosion of a cast stainless steel pump rotor (Type AISI 316) in a mixture of halogenated hydrocarbons of not specifically characterized composition is demonstrated in Figure 3. Erosion and crevice corrosion combined with t-stress corrosion cracking has been found on a stainless steel pump wheel in a mixture of formic acid, trionane, methanol, and water (Heitz, 1974). Quite a number of unexpected effects, however, are found if kinetic data are compared. This is exemplified by the system Cu/organic solvent H2S04 (Figure 4) It can be seen that (a) corrosion rates in ethanolic solutions are larger than in aqueous solutions; (b) corrosion rates in acetone solution are nil; and (c) sulfuric acid acts as an inhibitor in acetic acid. To explain such facts, there is a need for a thorough discussion of the principles involved.
+
Corrosivity of Organic Solvents To systematize the vast field of possible corrosion systems, it is useful to apply three classification principles for the medium phase 0019-7890/78/12 17-0037$0l.OO/O
(a) Protic-Aprotic Systems. Examples of protic solvents are alcohols (ROH), carboxylic acids (RCOOH), and amines (R-"2). Aprotic solvents are hydrocarbons (R-H), esters (RCOOR), and halogenated hydrocarbons (R-X) (b) One-Component/Multicomponent Systems. Onecomponent systems are usually ROH, RCOOH, R-X. In many cases, however, one or more organic solvents with the contaminants HzO, 0 2 , inorganic acids, halogenides, etc, are present. (c) One-Phase/Multiphase Systems. A one-phase system can be a vapor (Le., RH, RX), a liquid (in most cases), or a solid (coatings, polymers). In this paper only liquid phases are discussed. Multiphase systems may be encountered: heptane C2HsOH HC1; dioxane H2O HC1; RX H2O. From these classification principles three main groups of corrosion systems are derived.
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+
+
+
+
Protic Solvents-One or More-Component SystemsElectrochemical Mechanism Main representatives are alcohols and carboxylic acids. Most interesting is the influence of the number of C atoms in a homologous series which is shown in Figure 5 for the series of monocarboxylic acids. I t is evident that the corrosion rate decreases exponentially with increasing chain length of the carboxylic acid. This finding can be explained by the fact that with increasing chain length, steric hindrance and viscosities increase while the diffusion coefficients decrease. Figure 6 is an example of a two-component system, representing again the influence of the number of C atoms in the case of the alcohol series, with the same characteristics. It is interesting to see that methanol and ethanol show a higher reactivity than water. A more complicated, three-component system (formic acid water, C1-) is shown in Figure 7. The corrosion of 18-8 CrNi-steel increases with increasing C1- and water concentration. From experimental evidence it may be safely assumed that reactions of metals with protic media follow an electrochemical mechanism. The anodic partial reaction is the dissolution of a metal according to the equation
+
+
Me
-
Menf
+ ne-
and the cathodic partial reaction, Le., the reduction of acidic hydrogen of a proton donor, is represented by HA
+ e-
-
I/2H2
+ A-
Important is the fact that most of the processes involve the direct reaction of the nondissociated proton donor with the metal. 0 1978 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., VoI. 17, No. 1. 1978 39
FeinL%HCI+H20+Dioxane
mACmztlQo
figure 9. Current density potential cwves of the system Feldioxane t HCI + H20. Figure 6.Corrosion rates of some common metals in alcohols of different chain length with 0.01 N HCI.
02
1
lcOrr
CrNi 18 8- Steel in HCOOH 99 - 101' C. Nitrogen
Figure 7. Corrosion of CrNil8-8-steelin formic acid; HzO and CIeffect.
- -.
Figure 10. Initial state of the corrosion reaction of 99.5 A I in dibromoethane at hp (magnification,1:15).
L
-. .
.
~.. .
Y."QO
Figure 8. Corrosion rates of some stainless steels and nickel base dloys in the system DMF + water + 0.05 N HCI.
Figure 11. Noncorroded and corroded 99.5 AI specimen in dibromaethane at bp; reaction time, 35 min (magnification, kl).
Contrariwise, water acts as a stimulator in the reaction of steel with halogenated hydrocarbons as will be seen below.
etc.) and of high reactivity (aluminum and magnesium and its alloys). Purified aprotic solvents may only react with electronegative metals (AI, Mg, and its alloys) according to a radical mechanism in the sense of a metal-organic reaction. Main representatives of this class of compounds are halogenated hydrocarbons. With ferrous metals and in the presence of
Aprotic Solvents-One- and Two-Phase SystemsRadical and Electrochemical Mechanism I t is useful to divide the experimental material into corrosion cases of metals with medium reactivity (ferrous metals
40 I
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978 f
--D-
I
100 ,ipm H,O
550 ‘iom H,O
Figure 12. Induction time for the reaction A1 temperature and water effect.
.wj 105
CCli
+ dibromoethane; CH3CCI-j
slab.
n
Figure 14. Penetration rates of 99.5 A1 in different halogenated hydrocarbons at bp.
t ldl
Figure 13. Corrosion rates of the system AlMg4.5Mn/CC&at bp.
traces of water, a hydrolysis reaction under formation of hydrohalogens precedes the corrosion reaction which is electrochemical in nature. Corrosion Behavior of A1 and Its Alloys. Corrosion of A1 exhibits characteristic phenomena of a radical mechanism, Le., pronounced temperature dependence, large induction times, and, after the reaction has started, extremely high reaction rates. An example of this type is the reaction of 99.5 A1 with dibromoethane a t the boiling point. The corrosion begins at a cutting edge and develops pits (Figure 10). After 35 min reaction time the corroded A1 specimen has the appearance shown in Figure 11.The corrosion products of this exothermal reaction are AlBr3, HBr (if water is present), and polymers. The exact mechanism and the intermediates are not yet known, but are the objects of present work. As mentioned above, the reaction of A1 in halogenated hydrocarbons exhibits an induction time. Its dependence on water content and temperature in the case of dibromoethane is shown in Figure 12. It is postulated that during this induction time, the passive layer of AI is being destroyed by a chemical process. The induction time varies form a few seconds (Figure 12) to many days (Figure 13) (Dolling, 1977), depending on the nature of the halogenated hydrocarbon and the composition of A1 alloy. Generally Mg orland Mn cause a longer induction period (Stern and Uhlig, 1952). A summary of the measured penetration rates of 99.5 A1 in different halogenated hydrocarbons is shown in Figure 14. It can be concluded that the reaction rates are orders of magnitudes higher than those of common corrosion reactions. They give strong evidence to rate determining purely chemical steps.
Figure 15. Corrosion behavior of stainless steel CrNiMo 17-4-4 in trichloroethylenewith and without stabilizer;vapor phase, 74 “C.
Corrosion Behavior of Ferrous Metals. Besides localized corrosion as shown in the case histories section, uniform corrosion of nonalloyed iron and steel is widespread. This phenomenon has much in common with atmospheric corrosion. Of interest is the dependency of corrosion rate on water content which is analogous to the concept of “critical relative humidity” in atmospheric corrosion (Chistiokov, 1965). According to this theory water causes hydrolysis of halogenated hydrocarbon as soon as a condensed layer is formed at the metal surface. The condensation process is a function of water content and temperature. Among hydrolysis products, H X (X = C1, Br . . .) have been found to be the main cause of corrosive attack. In this regard the effect of stabilizer should be mentioned. Most of the halogenated hydrocarbons are commercially provided with small amounts of organic substances (commonly amines and alcohols) which prevent autoxidation and may inhibit corrosion attack. The influence of stabilizer is demonstrated in Figure 15 for the case of CrNiMo 17-14-4 N-alloyed stainless steel in trichloroethylene (Vehlow, 1977). The results have been obtained by the radionuclide technique for determination of corrosion products. Utilizing this method, the concentration of corrosion products from an irradiated specimen can be followed by measuring the activity increase of the medium. Summarizing experimental results, the following order of corrosivity of halogenated hydrocarbons in contact with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
41
CCI*:CC12
I'tt
stab.
,
prot c
HL".
HA
,
Apro tic
+
e-
t @-
Oxsdu + eRX + H2O
'i H2
5 H,
lcathodtc, t
A-
Red&, ROH + HX
Me + RX RMeX RMeX + R X MeX, t R - R (R=alkyl,oryl X = CI.Br.1 Me t RX, MeX, + -RZAIBr, + 31-C,HL-II [2Al + 3C,H,Br,
1 5% @J
hydrolysis
r a d i c o
1
Figure 17. Reaction steps and mechanism of base metals in organic solvents.
Figure 16. Corrosion rates of C-steel in different halogenated hydrocarbons at bp.
nonalloyed steel is given in Figure 16. Both nature of solvent and stabilizer content influence corrosion rate.
Conclusions The most important equations for the reaction mentioned above are summarized in Figure 17. The basic difference between electrochemical and chemical mechanism is evident. The main conclusions can be stated as follows. 1. Corrosion is mainly induced by protic solvents, even if they are present in low concentrations. 2. Aggressive agents may develop different corrosion rates, depending on the protic action of the solvent. 3. Water may act as stimulator or as inhibitor. In the first case, i.e., nonalloyed iron in halogenated hydrocarbons, the water content has to be as low as possible; in the second case, Le., A1 in methanol, water has to be present a t least at a concentration of 1%. 4. With A1 and its alloys prolonged induction times with
subsequent high corrosion rates may exist; these induction times are temperature dependent. The duration of tests has to be adjusted to this effect. 5. Stabilizers should inhibit corrosion besides autoxidation; a t the present time they are not optimal with respect to corrosion inhibition.
Acknowledgments The authors would like to thank H. Dolling, Bonn, J. Vehlow, Karlsruhe, and W. Deuchler, Pegnitz, G.F.R., for having provided experimental material and case histories. Financial support of the work by the Bundesministerium fur Forschung and Technologie is gratefully acknowledged. Literature Cited Chistiakov. V. M., Zh. Prikl. Khim., 38, 1021 (1965). Demo, J. J., Chem. Eng. World. 7, 115-124 (1972). DBlling, H., Forschungsinstitut. VAW, Bonn, G. F. R. (private communication), 1977. Heitz, E., Adv. Corros. Sci. Techno/., 4, (1974). Heitz, E., Czepurnyi, T., Proceedings. 6th InternationalCongress on Metal Corrosion, Sydney, in press, 1977. Stern, M., Uhlig, H. H., J. Electrochem. SOC.,99, 381-388 (1952). Vehlow, J., lnstitut fur Isotopentechnik, Kernforschungszentrum Karlsruhe, G. F. R. (private communication), 1977.
