1.  Field of the Invention
This invention relates generally to novel thiazolidines, the method for making these thiazolidines and the use of the thiazolidines for corrosion inhibition. 
2.  Background of the Invention
Various sulfur-containing compounds have been used as corrosion inhibitors for many different types of applications.  For example, heterocyclic sulfur compounds such as 4-neopentyl-5-t-butyl-1,2-dithiole-3-thione have been used as corrosion inhibitors, particularly in an environment where metal failure through stress cracking is a concern.  Another compound, thioglycolic acid, in combination with imidazolines, has been used for North Slope operations in Alaska.  While being effective against corrosion, this formulation is quite expensive and has a strong unpleasant odor. 
U.S. Pat.  No.  5,197,545, assigned to the assignee of this invention, discloses a novel method for volatile inhibition of corrosion and transportation of corrosion inhibitor in a gas lift process in which a gas is pumped into a well to facilitate petroleum production of the well.  The corrosion inhibitors used are 2,5-dihydrothiazoles of the following formula: 
R, Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 and R4 are, independently, H, methyl, ethyl, n-propyl or isopropyl and the total carbon number of these five groups does not exceed about 14.  One such compound, 2,5-dihydro-5,5-dimethyl-(1-methylethyl)thiazole, i.e. R=CH(CH3)2, Rxe2x80x2=CH3, Rxe2x80x3=CH3, Rxe2x80x2xe2x80x3=H, and R4=H, is referred to hereinafter as compound A1.  These 2,5-dihydrothiazoles are prepared by methods described in U.S. Pat.  No.  4,477,674. 
While these dihydrothiazoles have good solubilities in hydrocarbons and/or sufficient vapor pressures for both liquid and vapor phase corrosion inhibitions, they do not have high solubilities in systems such as brine, where aqueous corrosion inhibitions are needed.  Thioglycolic acid does have sufficient solubility in aqueous systems, but as mentioned before it has a highly unpleasant odor, which makes it less desirable.  As a result, in applications where brine is present, there is a need for inhibitors that have low odors and sufficient solubilities in various aqueous systems. 
It was unexpectedly discovered that a number of novel N-substituted thiazolidines and their mixtures can be prepared by reacting dihydrothiazoles with a mixture comprising formic acid and formaldehyde under effective conditions. 
The present invention relates to a composition which consists essentially of an N-substituted thiazolidine and mixtures thereof with a structure disclosed herein (structure B below) in the detailed descriptions of the invention section. 
It is another object of the present invention to provide a method for preparing such N-thiazolidines by reacting dihydrothiazoles with a mixture comprising formic acid and an aldehyde. 
It is a further object of the invention to use the new thiazolidines as corrosion inhibitors.