HIGH TEMPERATURE CORROSION INHIBITOR FOR CHELATING AGENT
The present invention relates to corrosion inhibiting composition, particularly high temperature corrosion inhibitor for chelating agents which are involved as cleaning solvent.
Alkaline salt solutions of organic chelating agents are conventionally employed for chemical, cleaning of boilers and heat exchangers. During the cleaning process, the iron oxide (Fe2O3), magnetite (Fe3O4), and/or copper (Cu) and/or copper oxide (Cu2O) are removed and the metallic surfaces gets finally passivated or deactivated so that further corrosion on such surface is prevented.
The present invention relates to a new, efficient and reliable inhibitor which prevents the metal loss during the above chemical cleaning of boilers and heat exchangers when the metallic surfaces are contacted by alkaline salt solutions of organic chelating agents at their boiling temperatures namely ranging between 100°C and 160°C with corresponding pressure from 1 kg cm"2 (g) to 5 kg cm"2 (g).
Normally, when a steel surface is contacted by an inhibited alkaline salt solution of organic chelating agent e.g. alkaline salt of alkaline diamine tetraacetic acid at temperatures between 100°C and 160°C, there will be heavy metal loss, ranging from 55 g m 2 h1 to 70 g m 2 h1.
But if the contact of the cleaning solution with the metallic surface under the aforesaid condition is made in presence of the new inhibitor of the present invention, the metal loss is limited to less than 1.0 g m "2 h"1.
The inhibitor content in the cleaning solution varies between 0.005 and 3.0 weight percent.

The inhibitor of the present invention is an intimate mixture of a polyalkanolamine, a polynuclear heterocyclic compound containing sulphur and a reaction product obtained by the combination of a nitrogeneous compound containing an active hydrogen atom attached to nitrogen, a primary amine, an aldehyde and a carboxylic acid.
It has been found advantageous to fit in a non-ionic surfactant in the inhibitor composition to get clarify while mixing the inhibitor with the cleaning solution of operation. The addition of non-ionic surfactant is only optional.
The temperature of decomposition of the inhibitor of the present invention is 320°C. Hence this inhibitor can be safety applied for liquids operating at high temperatures like cleaning solutions held inside a fired boiler operating at less than 5 kg cm"2 (g), the temperature of the cleaning solution being less than 160°C at which the corrosion rate of the carbon steel is falling within the acceptable limits of 1.0 g m "2 h"1.
In the preparation of the inhibitor of the present invention, a nitrogen compound containing at least one active hydrogen attached to the nitrogen atom per molecule is condensed with a primary amine, an aldehyde, and excess carboxylic acid.
The reaction medium is a solvent preferable a poly alkylene glycol.
The temperature of the reaction is from 60°C to 130°C and the reaction period is from 0.5 to 30 hours.
The condensation adduct obtained is then mixed with a mixture containing a polyalkanolamine and a polynuclear heterocylic compound containing sulphur to result in the inhibitor of the present invention.

Brief Description of the Invention
A preferred embodiment for preparing the inhibitor of the present invention is to first prepare the condensation adduct (A) by reacting about 1 mole of active hydrogen compound and from 1 to 10 moles of aldehyde and from 1 to 10 moles of a primary amine and from 0 to 5 moles of concentrated sulphuric acid catalyst if needed, with (B) from 0.1 to 30 moles and preferable from 0.1 to 2.0 parts by weight based on reactant (A), of a carboxylic acid at a temperature from about 60 C to 130 C for 0.5 to 30 hours, the,entire reaction being conducted in the medium of a polyalkylene glycol.
After the condensation reaction is completed, a further quantity of carboxylic acid is optionally added with stirring to build up the concentration of the carboxylic acid to fromO. 1 to 30 parts by weight of the acid per part of the condensed adduct.
Additionally, the non-ionic surfactant in an amount of about one part per 1 to 15 parts of the above product can be added with stirring.
Finally, the mixture obtained as above is to be further mixed with from 0.1 to 25 times its weight of a mixture of polynuclear heterocylic compound containing sulphur and a polyalkanolamine in the weight ration from 1:1 to
1:5.
The method described as above results in the inhibitor of the present invention.
The active hydrogen compounds which can be employed to prepare the condensation adduct (A) which forms a component of this present invention are those organic ammonia derivatives having atleast one hydrogen atom attached to nitrogen, as for example, urea, diamines, aromatic amines, alicyclic amines, polydecyl amines, eicosylamine, polyalkylamines etc. Thus one can employ thiobiurets, formamide, morpholine, and as well as irnidozolines.

The class of aldehydes which may provide the source in accordance with the
present invention include the aldehydes having from 1 to 17 or more carbon
atoms. Thus one can employ formaldehyde, acetaldehyde, urotropine,
hexadecanal and the like.
The primary amine used in the present invention is from a class of
nitrogeneous compounds having more than 5 carbon atoms.
The class of fatty acids found to be operative include the alkyl carboxylic acids
having from 1 to 25 carbon atoms , olefinic fatty acids with 2 or more
unsaturated sites along the chain.Thus one can employ abietic acid, capric
acid, coco fatty acids, lauric acid, formic acid , ethoxylated fatty acids, and the
like.
The wetting agents which can be employed with the inhibitor of the present
invention are those selected from the classes known as anionic, non-ionic,
cationic and amphoteric. Some of such wetting agents are the ehoxylated
alkyl phenols.
The acid catalyst employed in preparing the inhibitor of the present invention
are mineral acids such as sulphuric, hydrochloric .phosphoric and strong
organic acids such as formic, acetic, propionic acids and the like.
The sulphur containing compound which is employed in the present invention
is an organic heterocyclic sulphur compound having sulphur in the form of
—S— or S = The compound which fall within this definition are thiophene,
the mercaptothiozolines, the mercaptans, the mercaptobenzothiazoles and
mixtures of two or more members from the same or different classes.
The inhibitor of this present invention has several advantages over the known inhibitors currently being employed during acid cleaning of boilers.
1. Better inhibitive action with no compromise on the higher cleaning power of
the alkaline salt solution used, e.g. alkaline salt solution of alkylene
diamine tetra acetic acid, alkaline salt of citric acid etc.
2. The inhibitor of the present invention does not leave any residual solids on
metallic surfaces, as do many other inhibitors.

3. The inhibitor contains nil chlorides and hence it is advantageous for use in
both fossil and nuclear boilers.
4. Other materials like copper, zinc etc. are not attacked by the inhibitor of the
present invention.

We claim:
1. A corrosion inhibitor consisting essentially of (A) a condensation adduct prepared by
(1) Reacting about 1 mole of a compound containing 1 to 3 numbers of
active hydrogen atoms selected from the group consisting of an N-alkyl
acetamide, a polynuclear Hydrazide, Hydroxamic acid and N-substituted
alicyclic amine.
(2) from 1 to 10 moles of an aldehyde selected from the group consisting
of abietyl-, dihydro abeityl aldehydes, formaldehyde and allyl aldehyde and
mixtures thereof
(3) from 1 to 10 moles of a primary amine selected from the group
consisting of nondecylamine, Isoamyamine, dipropyiamine,
cyclohexylamine, octylamine, didecylamine and propylenediamine and
mixtures thereof.
(4) from 0 to 5 moles of concentrated sulphuric acid catalyst if needed
(B)
from 0.1 to 30 moles and preferably from 0.1 to 2.0 parts by weight based on
reactant (A) of a carboxylic acid selected from a group consisting of citric
acid, acetic acid, formic acid, dimethyl acetic acid, and hydroxy acetic acid
and all dissolved or suspended in a polyalkylene glycol of molecular weight
from 60 to 300,
obtained by reacting the aforenamed components A and B at from 60°C to
130°C for 05 to 30 hours.
(C)
from 0 to 2 part of a non-ionic surfactant having from 5 to 50 ethoxy groups
and
(D)
from 0.1 to 2.5 times its weight of [A + B] of a poly nuclear heterocyclic
compound containing sulphur selected from the group consisting of
mercaptothiazoline, sodium mercaptobenzothiazole, 2,4- Dimethyl
mercaptobenzothiazole, and mixtures thereof and polyalkanolamine in the
weight ratio from 1:1 to 1:5.
inhibitor as claimed in Claim 1 wherein
2 A corrosion/inhibited acid consisting essentially of an aqueous solution of
an acid selected from the group consisting of sulphamic acid, hydroxy acetic acid, citric acid, ethylene diamine tetra acetic acid, and partially ammoniated ethylene diamine tetra acetic acid having a pH from 9.0 to 10.0 and from about 0.005 to 3.0 weight percent of the corrosion inhibitor.
3 A process for preventing corrosion of metal in contact with an aqueous
acid solution, which comprises contacting the metal surfaces with an aqueous
solution of an acid selected from the group consisting of sulphamic acid,
hydroxy acetic acid, citric acid, ethylene diamine tetra acetic acid, and partially
ammoniated ethylene diamine tetra acetic acid having a pH from 9.0 to 10.0
and from about 0.005 to 3.0 weight percent of the corrosion inhibitor,
as claimed in Claim 2 or 3
4 A process /or preventing the corrosion of metals in contact with an aqueous
solution of an alkylene diamine tetra acetic acid ammoniated to a basic pH
from 9.0 to 10.0, which comprises contacting the metal surfaces with said
aqueous solution consisting essentially of said aqueous solution and from
about 0.005 to 3.0 weight percent of the corrosion inhibitor,