Metal Oxide Remover Containing a Strong Mineral Acid, Citric Acid and a Basic Ammonia Derivative

1. Field of the Invention

This invention relates to a composition and method for removing metal oxides such as rust and mill scale from ferrous metals such as steel. More particularly, the invention relates to metal oxide removal from ferrous metals while avoiding corrosion and discoloration of the metal.

2. Description of the Prior Art

It is known to use citric acid and citrates for the removal of rust from ferrous metals, see for example U.S. Pat. No. 3,510,432. The disadvantage of such materials is that they leave a discoloration or black film on the rust-free metal.

Another reference teaching the use of citric acid is U.S. Pat. No. 3,492,238 which employs citric acid in combination with EDTA (ethylene diamine tetracetic acid). This reference employs the solution at a pH of about 6.0-7.0.

Triethanolamine has been employed in metal cleaning formulations as an inhibitor for acid attack on the metal substate. It has been employed to avoid the blackening or discoloration of the cleaned metal. In this connection, reference is made to U.S. Pat. No. 1,723,923 which teaches the combination of triethanolamine in highly corrosive pickling baths such as cold concentrated sulphuric form or a heated more dilute form. Such pickling baths are corrosive to the plant in which they are used and present an ecologically unacceptable disposal problem.

U.S. Pat. No. 3,095,379 describes a metal cleaning composition which is the high temperature reaction product of citric acid and monoethanolamine. Such a formulation has been found to also leave an undesirable black coating on the cleaned metal surfaces.

Other references considered include the following: U.S. Pat. Nos. 2,006,216; 2,505,785; 2,994,664; 3,056,746; 3,282,848; 3,510,432; 3,589,895; and 3,779,935.

Attention is invited to the parent of this application referenced above for the citation of additional prior art.

The present invention provides a formulation which leaves the ferrous metal surface substantially free of metal oxides and substantially free of discoloraton such as the blackened condition formed by various of the prior art formulations. In addition, the present cleaning solution is essentially non-corrosive and does not attack either the metal being cleaned or the equipment utilized in the cleaning operation. It is thus highly acceptable from an ecological standpoint.

All of these advantages are accomplished with a composition comprising an aqueous solution containing a basic ammonia derivative selected from ammonium hydroxide and organic amines, citric acid and a strong mineral acid, all of said components being present in effective concentrations to remove metal oxides from the metal to be cleaned in the absence of acid corrosion and discoloration thereof, the pH of said solution being about 0.5-3.0 and the weight ratio of said ammonia derivative to said citric acid being about 2:7 to 7:2.

It will be appreciated that the components used will form ions in aqueous solution. Accordingly, equivalent results can be obtained by addition of the various components as salts which form the desired ions. For example, an amine citrate salt could be used to supply part of the citric acid and organic amine components.

It is essential to the efficient operation of the formulation that the pH and component ratios be maintained within the above limitations. In a preferred embodiment, the formulation pH is about 1.0-2.0, most preferably about 1.5. In all cases an effective amount of strong mineral acid to achieve clean surfaces should be present. A typical formulation for removing metal oxides from ferrous metals has the following formula in approximate parts by weight, said formula being adapted for use in a concentrate or for dilution with additional water: 30 water, 2-7 basic ammonia derivative, 7-2 citric acid, and at least about 0.25-0.5 of strong mineral acid, said formula havin a pH of about 1-2.

The basic ammonia derivative employed will either be ammonium hydroxide or an organic amine. Any water soluble amine is contemplated including aliphatic and aromatic amines. Examples are alkyl amines, alkanol amines. The amine may be primary, secondary, tertiary or quaternary in structure.

As an optional additive the formulation may include an organic cationic corrosion inhibitor of the type designed to inhibit the attack of hydrochloric acid or sulphuric acid on ferrous metals.

Examples of some usable formulations in aproximate parts by weight are next given. These formulas are adapted for use in the concentration shown or they may be diluted with additional water as desired and as will be illustrated in the working examples.

64. 0 Water

10.5 Triethanolamine

10.5 HCl Acid 20.degree. Baume

15.0 Citric Acid

64.0 Water

10.5 Tirethanolamine

10.5 H.sub.2 SO.sub.4 -66.degree. Baume

15.0 Citric Acid

Where an optional organic cationic corrosion inhibitor is desired it may be added to the above formulas in the amount of about 1 ounce per gallon of the formulation. For example, in formula A a suitable additive is the commercial corrosion inhibitor available from Amchem Products Inc. and offered under the trade name "Rodine 213". With respect to Formula B, an appropriate corrosion inhibitor for sulfuric acid available from the same company is known as "Rodine 92A".

The above Formulas A and B are believed to be useful formulations and concentrations for many applications. It will be appreciated that the precise concentration of the components is subject to some variation from that shown in the formulas. It is contemplated that each of the components may vary by as much as .+-.20% from the figure shown, provided that the final formulation is operative to remove metal oxides without corroding and discoloring the metal to be cleaned. The following Table I will illustate the effectiveness of the above type of formulations in ability to remove metal oxides while leaving the metal clean and free from corrosion and discoloration. Data was obtained by the following procedure.

7g of citric acid in 30g of water was neutralized by the following materials: triethanolamine, diethanolamine, monoethanolamine and ammonia. The pH was adjusted to 3.5 with concentrated HCl. DEX and the material from U.S. Pat. No. 3,510,432 were purchased, while U.S. Pat. No. 3,095,379 was followed to produce Example I and Example A. Citric acid was run straight in water (7g in 30g water). All examples were placed in 100ml beakers filled to the 30ml mark and pieces (1/2" .times. 2") of rusty 18 gauge 1020 cold rolled steel placed in them. The results at room temperature and 210.degree. F. are shown in Table I.

The formulations of this invention were tested in a commercial operation in which rusty 55 gallon drums were being cleaned. In this operation the cleanimg solution was applied to the drums by spraying with a nozzle at a pressure of 60 psi. For rapid operations it is desirable to employ the cleaning solution at an elevated temperature, for example at about 120.degree.-212.degree. F. to shorten treatment time. To be acceptable in this test operation it was considered that all rust should be removed with the spray within 3 minutes while the cleaned wet drums should not re-rust within 30 minutes. As will be seen, the present formulations met these conditions.

In this test 30 gallons each of Formulas A and B listed above were used, including the optional applicable Rodine corrosion inhibitors in the amounts of 1 ounce per galon of solution. The concentrates of Formulas A and B were diluted for use with water in the amount by volume shown in the example below. After spraying with the metal oxide remover solution, a rinse was applied as indicated. The results are as follows.

To illustrate the criticality of weight ratio of the ammonia derivative to the citric acid in the formula, the following experimental work was performed.

A series of solutions were prepared in 100ml beakers, each beaker contained the amounts shown in the Table below. In each case the beaker contained 30 g of water and the contents were adjusted to a pH of 1.5 with the addition of HCl. 1.times. 1 square inch pieces of rusty drum steel were placed in the solutions at boiling temperatures of approximately 212.degree. F. for 3 minutes. The results are shown in Table III. The first number at the head of each column refers to the amount of amine utilized and the number on the right at the head of each column refers to the amount of citric acid.

It will be observed that Hyamine 3500 was utilized as an 80% solution. Accordingly, the amount of active ingredient is somewhat different than the ratios indicated at the tops of the columns. The results with this particular and preferred quaternary amine point up the fact that the weight ratio does not have a sharp dividing line at the lower and upper limits. The weight ratios indicated throughout this specification should be taken as being approximate ratios subject to some variation on the order of .+-.20% as previously discussed in connection with Formulas A and B.

This example will illustrate the criticality of the pH in the formulations.

Solutions were made up using the amounts shown in Table IV. 100 ml beakers were used. 1.times. 1 inch (approximate) pieces of rusty drum steel were placed in boiling solutions at approximately 212.degree. F. for 3 minutes. The results are shown as follows.

This example will illustrate the requirement of the presence of a minimum amount of strong mineral acid in order to obtain the desired results. In particular, about 0.25-0.5g or more of strong mineral acid is required in a concentrate formulation containing 30g of water and the weight ratios of the other components shown in Example 1. This point was established as follows.

In a 100 ml beaker 7 g of citric acid was added followed by 30 g of HOH. Enough triethanolamine was added to give a pH of 3 (3.5 g approximately). A piece of 1" .times. 1" rusty drum steel was added to the boiling material for 3 minutes. The result was recorded at "A" below. Water was then added to replace the amount boiled out and 0.25 g of concentrated HCl added followed by enough triethanolamine to adjust the pH to 3. Again a rusty 1" .times. 1" piece of drum steel was added to the boiling solution for 3 minutes. "B" was the result. Finally after adding lost water, 0.5 g of concentrated HCl was added and enough triethanolamine to bring the pH back to 3. As before a 1" .times. 1" rusty piece of drum steel was added to the boiling solution for 3 minutes. The result was recorded at "C".

This example illustrates that any strong mineral acid may be utilized. The procedure followed was similar to Example 3. Thus, four solutions were made up in 100 ml beakers. Each beaker contained: 7 g citric acid, 3.5 g triethanolamine, 30 g HOH. The pH was 3.

In the first instance no strong mineral was added. In the other three beakers concentrated mineral acids as indicated were added and additional triethanolamine was thereafter added to bring the pH back to 3. In each case a 1" .times. 1" sample of rusty drum steel was treated for 3 minutes at boiling temperature. The results were as follows:

With respect to the amounts of mineral acid added, reference has been made to the minimum of 0.25-0.5g. This amount refers to the usually encountered concentrated form of the acid. For example, in the case of HCl the concentrated solution is 37% in strength. 0.25-0.5 g of this concentrate is the minimum amount referred to. Similarly, in the case of phosphoric acid the 0.25-0.5g refers to the 85% concentrate of phosphoric acid and 98% with respect to sulfuric acid.