US3050360A

US3050360A - Method of inhibiting corrosion of iron and steel

Info

Publication number: US3050360A
Application number: US56669A
Authority: US
Inventors: Roos Huib De; Willem F Jense
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1959-11-26
Filing date: 1960-09-19
Publication date: 1962-08-21
Anticipated expiration: 1979-08-21
Also published as: BE597460A; DE1185450B; GB888682A; NL245796A

Description

3,050,360 Patented Aug. 21, 1962 3,050,360 METHOD OF HJHIBITING CORROSION F IRUN AND STEEL Huib de Bees and Willem F. Jense, Amsterdam, Netherlands, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Sept. 19, 1960, Ser. No. 56,669 Claims priority, application Netherlands Nov. 26, 1959 1 Claim. (Cl. 21-2..7)

The invention relates to the inhibition that is to say the prevention or reduction, of the corrosion of iron and steel, especially the corrosion of iron and steel which comes into contact either continuously or intermittently with water containing chlorine ions. In this specification the term iron and steel is to be understood to include alloys of iron. The invention is particularly important with respect to the metal surface of cargo tanks in marinetankers.

The walls of the tanks are almost continually exposed to corrosive influences. Petroleum or petroleum products transported by these tankers may cause corrosion on account of the great solubility of oxygen in these products and the small amount of water which is usually present in such a cargo.

Serious corrosion is also caused by the periodical cleaning of the tanks which, in order to save time, is usually done at sea and consequently with sea-water. For this purpose the sea-water is often heated to 80 C. to 90 C. and then sprayed until pressure against the walls in order to remove as much rust as possible together with the oil residues. Finally, when the tank is empty the walls are corroded by the atmosphere, especially at sea.

In view of the losses sustained by contamination of the products transported, there is a more urgent need for effective methods of inhibiting corrosion in tankers than under other conditions.

When treating the Walls of the cargo tanks with corrosion-inhibiting materials, known as inhibitors, great care should be exercised in the choiceof the inhibitor. These materials cause turbidity or haze in the oil products transported even when present in small concentrations, as they promote the emulsification of water in the oil phase. Other inhibitors are soluble in oil and this is also undesirable. Moreover, it has been found that the effect of many proposed inhibitors is insufficient for the present purpose.

In the co-pending United States patent application No. 757,111, a method is described of inhibiting the corrosion of the walls of cargo tanks in tankers in which the walls are contacted with an aqueous solution of potassium ferrocyanide (11 to 24% by weight). This process is satisfactory, but it has been found possible to obtain a still better protection against corrosion by using a compounded inhibitor mixture.

It has now been found that the said improvement is obtained by treating the iron and steel with an aqueous solution of alkali metal nitrite or calcium nitrite, alkali metal acetate and an alkanolamine, the solution containing not more than 89% of water as well as 3-7 parts by weight of alkali metal nitrite or calcium nitrite to 2-6 parts by weight of alkali metal acetate to 2-5 parts by weight of alkanolamine.

The maximum effect is obtained by means of a solution containing 4-6 parts by weight of alkali metal nitrite or calcium nitrite to 3-5" parts by weight of alkali metal acetate to 34 parts by weight of alkanolamine. Outside the extreme limits specified there is a marked reduction in the corrosion-inhibiting effect.

The preferred alkali metal nitrite used is sodium nitrite and the preferred alkali metal acetate is sodium acetate, while the prefererd alkanolamine is trior di-ethanolamine or a mixture of water-soluble alkanolamines. A greater dilution of the solution than that specified above as the extreme limit, viz. with more than 89% of water, gives a reduced or inadequate protection, at least if the iron or steel has not been previously treated; concentrated solutions hardly lead to any improvement in the result. With the second or subsequent treatment of the iron or steel it is, however, possible to maintain the protective eifect by treating the surface of the metal from time to time with the solution of a much lower concentration which does not, however, preferably have a water content exceeding The surfaces to be protected may be treated with the above solutions according to any technique known in the art. Fairly large surfaces, for example, the walls of the cargo tanks in tankers may be sprayed with the solution. This treatment may be carried out at normal temperature, i.e. the ambient temperature, but if desired, also at elevated temperature, for example, 40 C.50 C. No other substances, for example, various phosphates, which are found in inhibitor solutions need be incorporated in the solution. The presence of chlorine ions in the solution does not neutralize the corrosion-inhibiting effect.

It has been found that excellent results can be obtained by using a concentrate solution containing 23 parts by weight of sodium nitrite, 17 parts by weight of sodium acetate and 14 parts by weight of tri-ethanolamine and 46 parts by weight of water, and reducing the concentrations by dilution with amounts of water to produce a treating solution containing 3-7 parts by weight of sodium nitrite, 2-6 parts by weight of sodium acetate and 2-5 parts by weight of tri-ethanolamine.

EXAMPLE I Steel strips containing 0.1% of carbon were treated in the following way.

The sand-blasted strips were immersed in synthetic sea-water for four half-hourly intervals spread over one day during two days. They were then suspended above synthetic sea-water for two days and then treated in the following way:

1st day: sprayed once with synthetic sea-water until they were just wetted;

2nd day: sprayed once with the inhibitor solution until just wetted;

15th day: immersed for one hour in kerosine in which 0.1 of synthetic sea-water had emulsified;

29th day: as for 1st day, etc.

Rate of corrosion Concentration of active material (reduction in (percent by weight) thickness) (mm. per annum) 3 The synthetic sea-water was prepared by mixing equal volumes of the following solutions I and II and by adjusting the pH values to 8 by adding a solution of 0.1 N NaHCO Solution I NaCl 24.5 KCl 0.69 KBr 0.10 Na]? 0.003 N21 SO 4.09 NaHCO 0.20 H BO 0.03

Solution II MgCl .6H O 11.10 CaCl 1.16 SIC12-6H20 0.04

EXAMPLE II Only the treatment schedule was modified, other details being the same as those of the first example.

In this case the treatment schedule is as follows:

1st day: sprayed once with synthetic sea-water until just wetted;

2nd day: sprayed once with a solution containing 13.5%

of active material until just wetted;

15th day: immersed for one hour in kerosine in which 0.1% by weight of synthetic sea-water had emulsified;

29th day: sprayed once with synthetic sea-water until just wetted, etc.

The cycle was repeated four times, i.e. to the 141st day but during the cycles succeeding the first cycle a solution was used which contained 5.4% of active material.

After 141 days the rate of corrosion was found to be 0.04 mm. per annum.

The method according to the invention has a number of considerable advantages over the methods of treatment of iron and steel with corrosion-inhibiting materials which have hitherto been known. Thus the present inhibitor is comparatively inexpensive and owing to the good solubility concentrated. solutions may be prepared which is advantageous in connection with transport and storage. Moreover, the solubility at low temperature is excellent so that there is scarcely any risk of crystallization. In addition there is no hazing of the oil products which have been in contact with the inhibitor solution. The solubility in hydrocarbons is also extremely low. Finally, the inhibitor has not been found to have any effect on the fuel properties of the fuel transported in treated cargo tanks.

We claim as our invention:

A corrosion inhibiting process for ferrous metal cargo compartments of a sea-going oil tanker which comprises: coating the metal inner surface of said compartment containing residual amounts of refined petroleum products with an aqueous concentrate containing 3-7 parts by weight of sodium nitrite, 2-6 parts by weight sodium acetate and 2-5 parts by weight of tri-ethanolamine and the balance to make parts by weight being water, said concentrate being diluted with water to give a total active material concentration in final dilution of in excess of about 5.4% by weight and said final aqueous composition being coated on said metal surface.

References Cited in the file of this patent UNITED STATES PATENTS Wighton et al. Nov. 21, 1939 OTHER REFERENCES