Claims
- 1. A method of inhibiting the stress corrosion of stainless steel in contact with aqueous and/or polar organic solutions containing Cl ions and optionally Cu ions which comprises adding to said solutions a member selected from the group consisting of quaternary ammonium alkylcarbonates and quaternary ammonium benzylcarbonates.
- 2. The method of claim 1 wherein the aqueous and/or polar organic solutions have a Cu ion and C1 ion concentration of not higher than 2 ppm and 20 ppm, respectively.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 23288 A/85 |
Dec 1985 |
ITX |
|
Parent Case Info
This application is a divisional of copending application Ser. No. 06/943,682, filed on 12/19/86, now Pat. No. 4,792,417.
The present invention relates to inhibitors of stress corrosion of stainless steels.
It is known that steels in general, and also stainless steels undergo stress corrosion, when they are in contact with some types of liquids or solutions.
It has been observed that the stress corrosion is particularly noxious in the case of Cu.sup.++ and Cl.sup.- ions, contact with solutions containging.
The possible solutions the art offers in the latter exposed case, which is the one we are mostly interested in, are either using stainless steels of the Hastelloy type, with a consequent increases in equipment cost, or reducing the contents of Cu.sup.++ and Cl.sup.- ions by means of a series of beds of anionic and cationic resins, with a consequent increase in plant operational cost both as it relates to the resins cost and consumption, and to the regeneration thereof.
It should be furthermore observed that the lower the concentration of Cu.sup.++ and Cl.sup.- ions, the more difficult is their separation by means of resins, so that, already starting from values of Cu.sup.++ and Cl.sup.- ions concentrations of 2 and 10 ppm, respectively the cost for such a separation would be prohibitive. It has been surprisingly found that the stress corrosion in the presence of Cl.sup.- ions, and possibly in the presence of Cu.sup.++ ions too, can be eliminated by resorting to corrosion inhibitors dissolved in the aqueous and/or polar organic solution containing said ions in contact with the stainless steel. Advantageously, the concentration of the current Cu.sup.++ and/or Cl.sup.- ions are first reduced by known means, in particular by such means as above mentioned, to maximum values of 2 ppm and 10 ppm, respectively preferably 1 and 5 ppm.
The corrosion inhibitors according to the invention are selected from the class of the quaternary ammonium alkyl or benzyl carbonates having general formula ##STR2## wherein: R.sub.1 is a linear or branched, saturated or unsaturated, possibly hydroxylated alkyl radical containing from 1 to 30 carbon atoms; R.sub.2 and R.sub.3 are alkylaryl radicals, in particular benzyl radicals, possibly bearing one or more substituents on their ring, or have, individually, the same meaning as R.sub.1 ; R.sub.4 is an alkyl radical of from 1 to 4 carbon atoms, or is a benzyl radical.
The carbonate used according to the present invention is obtained by means of a dialkylcarbonate having the formula: ##STR3## with respectively a tertiary or secondary amine having the formula: ##STR4## wherein: R.sub.1, R.sub.2, R.sub.3 and R.sub.4 have the above said meaning, in the liquid phase, at temperatures of from about 100 to about 200.degree. C., with an amount of carbonate equal to, or higher than the stoichiometric amount for the reaction with the amine, up to complete, or substantially complete conversion of the same amine.
The reaction between the dialkylcarbonate and the tertiary amine can be described as follows: ##STR5##
The reaction between the dialkylcarbonate and the secondary amine can be described as follows: ##STR6##
That is to say, the alcohol corresponding to radical R.sub.4 in the carbonate, as well as carbon dioxide, is formed.
Examples of dialkylcarbonates useful as alkylating agents are dimethylcarbonate, methylethylcarbonate, methylpropylcarbonate, methylbutylcarbonate, methylbenzylcarbonate, diethylcarbonate and dibenzylcarbonate.
Examples of tertiary amines useful for the purposes of the present invention are N,N-dimethylbenzylamine, trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, dimethylcetylamine and dimethylstearylamine. Examples of secondary amines useful for the purposes of the present invention are: laurylmyristylamine, dipropylamine, benzylcetylamine, dimethylamine, diethylamine, di-n-butylamine and benzylmethylamine.
The molar ratio between carbonate and amine is equal to at least 1/1 in case of tertiary amines and at least 2/1 in case of secondary amines. It is generally preferable to use an excess of carbonate relatively to the stoichiometric value, and, in practice, operating is possible with values of such a ratio of up to 10/1, with the values of from 3/1 to 5/1 being preferred. The possibly use of excess carbonate remains unchanged, and can be recovered for a subsequent use.
The reaction is carried out at a temperature of from about 100 to about 200.degree. C. and preferably of from 130 to 160.degree. C. and under such a pressure as to keep the reaction mixture in the liquid phase. The reaction conditions are a function of the nature of the amine, of the carbonate and of the solvent utilized. In practice, the pressure can vary from atmospheric pressure up to about 15 bars.
The reaction times depend on the nature of reactants used, and on the other conditions under which the reaction is carried out. Generally, under the conditions as set forth, the reaction is complete, or nearly complete, within a time of from 1 to 30 hours.
Furthermore, the reaction can be carried out in the presence of an added, not reactive, and preferably prior solvents. Solvents suitable for this purpose are the alcoholic solvents (in particular, methanol and ethanol), hydrocarbon solvents and ethereal solvents.
In order to achieve the highest reaction rate, should it be regarded as useful, a substance may be used, which performs a catalytic action on the formation of quaternary ammonium carbonates, such as organic and inorganic iodides, for example methyl iodide, ethyl iodide and sodium and potassium iodides. The catalyst can be used in amounts of from 0.1 to 5 mol per each 100 mol of amine, and preferably of from 0.5 to 2 mol per 100 mol of amine.
At the end of the reaction, the quaternary ammonium carbonate can be separated from the reaction mixture by simple filtration, when said product separates in the solid form at temperatures lower than the reaction temperatures.
As an alternative, the separation can be carried out by evaporating off the unchanged dialkylcarbonate, the solvent, if any as well as the byproduct alcohol.
The separation can be also simply accomplished by pouring the reaction mass into water and separating the carbonate excess, which is insoluble in the aqueous ammonium hydroxide solution.
The inhibitor concentration in the aqueous and/or polar organic solution containing Cu.sup.++ and Cl.sup.- ions is within the range of from 50 to 1000 ppm, preferably of from 100 to 600 ppm.
The corrosion inhibitors in accordance with the present invention allow, at the concentrations mentioned, austenitic, austeno-ferritic and superaustenitic stainless steels to be passivated, in a complete way, against the stress corrosion, when concentrations of Cu.sup.++ and Cl.sup.- ions are not higher than 2 and 20 ppm, respectively.
Should the values of concentrations of Cu.sup.++ and Cl.sup.- ions be higher than the above limits, the inhibitors of the present invention allow the stress corrosion to be reduced, but it is not to be completely eliminated.
The inhibitors of the present invention can be used in aqueous solutions, or in polar organic solutions, or also in water-polar organic liquid solutions or dispersions, with the maximum limit of concentration of Cu.sup.++ and Cl.sup.- ions being the only limitation.
Among the polar organic liquids, there should be mentioned the alcohols and, among these, in particular, methanol and ethanol; the ketones, and among these, in particular, acetone; and the esters.
We underline moreover that the activity of the inhibitors according to the present invention is in no way influenced by the presence, in the aqueous and/or organic solution, of organic compounds therein dissolved or dispersed, such as, e.g., esters, aldehydes or still others.
Some examples are now supplied for the purpose of better explaining the invention, it being understood that the present invention is not to be considered as being limited to them or by them.
US Referenced Citations (3)
Divisions (1)
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Number |
Date |
Country |
| Parent |
943682 |
Dec 1986 |
|
Patent Grant
4849170
