Corrosion inhibitor

Use of amino tris(methylene phosphonic acid) and 1-hydroxyethylidene-1,1-diphosphonic acid in a ratio of from about 1:1 to about 3:1 by weight to inhibit corrosion of low carbon steel in aqueous systems.

This invention relates to the inhibition of corrosion in aqueous systems. 
More particularly, this invention relates to the use of compositions 
containing amino tris (methylene phosphonic acid) and 
1-hydroxyethylidene-1,1-diphosphonic acid in a ratio of from about 1:1 to 
about 3:1 by weight to inhibit corrosion of low carbon steel in aqueous 
systems. 
Oxygen corrosion is, of course, a serious problem in any metal-containing 
aqueous system. The corrosion of iron and steel is of principal concern 
because of their extensive use in many types of industrial and municipal 
water systems. 
While amino tris(methylene phosphonic acid) and 
1-hydroxyethylidene-1,1-diphosphonic acid have been used to inhibit the 
corrosion of metals in aqueous systems, we have found that greatly 
improved results are obtained when compositions containing amino 
tris(methylene phosphonic acid) and 1-hydroxyethylidene-1,1-diphosphonic 
acid in a ratio of from about 1:1 to about 3:1 by weight are used to 
inhibit the corrosion of low carbon steel in aqueous systems. The 
compositions of this invention will effectively inhibit corrosion of low 
carbon steels when maintained in an aqueous system at a concentration of 
at least 0.1 mg/liter. The preferred concentration is at least 15 
mg/liter. 
Other conventional inhibitors such as inorganic polyphosphates, zinc, 
soluble zinc salts, chromates, benzotriazole, tolyltriazole or 
mercaptobenzothiazole may be added to the final formulation in varying 
amounts to improve its usefulness in a wider variety of industrial 
applications where both low carbon steel and copper or its alloys are 
present in the same system. Similarly, polymeric dispersants such as 
polyacrylates, polyacrylamides or polymers of 2-acrylamido methylpropane 
sulfonic acid may also be incorporated in the final formulation in varying 
amounts. The molecular weights of these dispersants may vary from as low 
as less than 1000 to as high as several million.

In order to demonstrate the effectiveness of the compositions of this 
invention, a coupon immersion test was conducted in a test system which 
consists of a cylindrical battery jar with a capacity of 8 liters. A Haake 
constant temperature immersion circulator (Model E-52) was used to control 
the solution temperature and agitate the controlled bath. The unit 
contained a 1000 watt fully adjustable stainless steel heater which 
permitted temperature control to +0.01.degree. C., and a 10 liter per 
minute pump with a built-in pressure nozzle agitator that ensured high 
temperature uniformity in the bath. A mercury contact thermoregulator was 
used as the temperature sensing element. The pH of the solution was 
controlled with a Kruger and Eckels Model 440 pH Controller. This unit is 
capable of turning power on and off to a Dias mini-pump whenever the pH of 
the corrosive liquid environment fell below the set point. The peristaltic 
Dias pump, with a pumping capacity of 20 ml per hour, maintained the 
solution pH with the addition of 10% sulfuric acid. Standard glass and 
saturated calomel electrodes were used as the sensing elements. The bath 
was continuously aerated at the rate of 60 cc per minute through a medium 
porosity plastic gas dispersion tube to ensure air saturation. Two 
SAE-1010 steel coupons, each having a surface area of 4.2 square inches, 
were suspended by a glass hook. The solution volume to metal surface area 
ratio for the larger beaker test was approximately 1000:1. 
The tests were conducted in water having a composition of 71 mg/liter 
calcium ion, 100 mg/liter bicarbonate ion, 224 mg/liter chloride ion and 
224 mg/liter sulfate ion. The system was treated with 15 mg/liter of 
corrosion inhibitor. After seven days, the water composition and inhibitor 
level was totally replenished; and at the expiration of fourteen days the 
tests were terminated. 
The corrosion rates shown in Table I are the average weight loss of low 
carbon steel coupons expressed in mils per year (m.p.y.). The coupons were 
prepared, cleaned and evaluated according to the ASTM method G1. 
The results of this test are reported in the following table. 
TABLE I 
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STEEL CORROSION INHIBITION 
Concentra- Tempera- 
Corrosion 
Inhibitor tion (mg/l) 
pH ture .degree. C. 
Rate (mpy) 
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1:1 AMP:HEDP 
15 7.5 50 5.0 
2:1 AMP:HEDP 
15 7.5 50 3.1 
3:1 AMP:HEDP 
15 7.5 50 9.4 
2.5:9 AMP:HEDP 
15 7.5 50 21.4 
AMP 15 7.5 50 18.0 
HEDP 15 7.5 50 27.1 
1:1 AMP:HEDP 
15 8.0 50 4.2 
2:1 AMP:HEDP 
15 8.0 50 2.9 
3:1 AMP:HEDP 
15 8.0 50 2.7 
2.5:9 AMP:HEDP 
15 8.0 50 6.7 
AMP 15 8.0 50 23.3 
HEDP 15 8.0 50 16.4 
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*AMP = amino tris(methylene phosphonic acid) 
*HEDP = 1hydroxyethylidene-1,1-diphosphonic acid