Method for inhibiting inland corrosion of steel

An improved method for inhibiting inland atmospheric corrosion of steel by a sacrificial coating of aluminum, galvanically activated by small surface deposits of metallic indium is disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to new and improved methods for inhibiting 
inland atmospheric corrosion of steel, and particularly to improved 
sacrificial coatings for steel substrates. 
In the past, ion vapor deposited aluminum has been proposed as a 
sacrificial coating to protect steel substrates from corrosion damage. 
Through salt fog exposure test such pure aluminum coatings have been shown 
to provide good potential for sacrificial corrosion protection of offshore 
steel substrates. However, evidence has been adduced to indicate that such 
pure aluminum coatings do not sacrificially protect inland steel 
substrates because the aluminum tends to passivate. 
Our co-pending application discloses a method for solving this problem by 
alloying aluminum with other metals, anodic to steel, such as zinc and 
indium in a co-deposition process. Although the codeposition of 
aluminum-zinc alloys has met with extreme success, co-deposition of 
aluminum-indium alloys leaves a great deal to be desired for commercial 
success. 
The periodical entitled "Corrosion" at Volume 40 , Number 7, July 1984, in 
the article "A Proposed Activation Mechanism For Aluminum Anodes" by the 
author's Reboul, Gimenez, and Rameau mentions that there is an exchange 
reaction between aqueous solutions of indium salts and aluminum to form 
metallic indium deposits on the surface of the aluminum. However, Reboul, 
et. al, believed that the indium had to be in solid solution in order to 
sufficiently activate the aluminum. 
Many indium salts are unstable in aqueous solutions because of their 
tendancy to form insoluble compounds each as the hydroxide. Nevertheless, 
Indium is a highly desireable source for activating aluminum if a process 
could be developed to incorporate it into aluminum coatings more 
effectively from aqueous solution than by co-deposition, and achieve 
greater stability of an aqueous solution of certain of its salts. 
Accordingly, a liquid solution activating process for aluminum coated steel 
substrates that would effectively permit sacrificial inland corrosion 
protection of steel in mild environments would be a surprising and 
unexpected advancement in the art, fulfilling a long felt need in the 
industry. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a new and 
improved method for inhibiting inland atmospheric corrosion of steel with 
a sacrificial aluminum coating activated by indium. 
It is a further object of the present invention to provide an improved 
indium activated aluminum coating for steel substrates without the need 
for producing a solid solution. 
It is also an object of the present invention to provide a new and improved 
method for activating aluminum coatings for steel substrates increasing 
the sacrificial corrosion protection mechanism with an aqueous indium salt 
solution which has been stabilized against premature precipitation of 
indium bases such as indium hydroxide. 
These objects and others will become more apparent from the following 
detailed description, examples, and claims. 
DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS 
In the process of the present invention pure aluminum feedstock is fed into 
conventional ion vapor deposition equipment. Such equipment is known for 
example from U. S. Pat. Nos. 3,750,623; 3,926,147; 4,116,161; and 
4,233,937. 
The pure aluminum is then ion vapor deposited onto steel substrates. The 
general parameters of such deposition process include, for example, 
aluminum wire feed rates between 3 and 40 inches per minute, depending 
upon desired deposition rate, wire diameter, chamber size, and total area 
to be coated; argon gas pressure between 5.times.10 and 5.times.10 torr; 
and an applied potential of between 1000 and 1800 volts between the parts 
to be coated and the evaporating boats. (The parts are at a negative 
potential with respect to the boats.) 
The aluminum coated steel substrates are then glass bead burnished to 
densify and cold work the deposited coating under the following conditions 
and ranges of parameters: glass beads are used as the burnishing agent 
under an air pressure of 30 to 40 psig in a standard abrasive blasting 
cabinet. 
In accordance with the present invention, it has been found that immersion 
of such aluminum coated and burnished parts in aqueous acid solutions of 
indium acid salt and holding the parts in such solutions will provide 
suprisingly, the requisite small surface deposits of metallic-indium 
needed for activation of the aluminum coating improving its inland 
sacrificial corrosion protection of steel. 
These indium solutions are prepared by dissolving indium salts such as, for 
example indium trinitrate, indium (III) sulfate, or indium trichloride but 
preferably indium trichloride. It is critical in the process of the 
present invention to prepare such solutions at acid pH ranging from about 
1.5, to about 3.0, but preferably about 2.0. The concentration of indium 
may be, for example, from about 0.08 to about 3.0, but preferably about 
0.1 molar of the preferred embodiment indium trichloride. One effective 
means for assuring the preferred pH of about 2 is to adjust the pH, with 
for example, hydrochloric acid. This acidification must be carried out at 
the time the solution is initially mixed. Once indium bearing precipitates 
are formed, they are difficult or impossible re-dissolve by addition of 
acid. Another reason for acidifying the solution is to aid in dissolving 
the naturally formed aluminum oxide passive layer on the aluminum coating. 
The bare and active aluminum surface is necessary to initiate the indium 
deposition. 
It has been discovered that it is also critical to the process of the 
present invention to hold or retain the aluminum coated steel substrate in 
the solution for a sufficient time. That time is preferably from about 2 
to 30 seconds. Lesser time will not permit sufficient deposition of 
metallic indium while retention times which exceed 30 seconds will 
adversely affect the process because of excessive deposition of indium. 
As previously stated, the effective amount of indium in the solution is 
provided within a molar amount of from 0.08 to about 0.25 molar. Lesser 
amounts are undesireable because of possible dissolution of excessive 
amounts of the aluminum coating, while greater amounts may be detrimental 
because of deposition of excessively large and unevenly distributed indium 
particles, or hydralyzed by-products. 
After holding the aluminum coated steel substrate in the solution for the 
requisite amount of time the steel is then removed from solution, rinsed 
by any convenient means such as water but preferrably by distilled water 
followed by a methanol rinse and subsequently drying. 
Surprisingly, the treated steel substrates exhibit activation of the 
aluminum and thus sacrificial corrosion protection to the steel even in 
inland environments without the need for a solid solution. Also 
surprising, is the fact that the indium is stabilized against premature 
precipitation in a basic form, such as indium hydroxide which would 
ordinarily be expected to precipitate because of hydrolysis reaction with 
water. The usual pH of an 0.1 molar solution of indium trichloride is 
approximately 3.8 at room temperature. Apparently, this pH is not 
sufficiently low to prevent hydrolysis and the resulting precipitation of 
indium hydroxide.

The examples which follow are intended for illustrative purposes and 
therefore, should not be construed as unduly limiting the scope of the 
present invention. 
EXAMPLE 1 
Four identical steel parts of the type normally used for atmospheric 
corrosion test specimens were ion vapor deposited with pure aluminum under 
the following conditions: At an argon plasma pressure of approximately 
10.sup.-4 torr and an impressed potential difference of 1200 volts between 
the specimens to be coated (negative) and the evaporation boats 
(positive). Each part was glass bead burnished to densify and coldwork the 
aluminum coating as follows: glass beads at approximately 40 psig in a 
standard sand blasting cabinet. One of the parts was labeled the control 
and the other three parts were labeled 1 thru 3. 
Twenty-one inches for scribe marks extending thru the thickness of the 
aluminum coating and exposing the steel substrate were made in control. 
A 0.1 molar solution of indium trichloride was produced and adjusted to a 
pH of about 2.0 with hydrochloric acid. 
Sample 1, was immersed in the solution and held for 1 second. Then it was 
removed, rinsed, and dried. Sample 2 was immersed, held for 5 seconds, 
removed, rinsed and dried. Sample 3, was immersed, held for 15 seconds, 
removed, rinsed, and dried. Each of the three samples, like the control, 
was intentially damaged by scribe marks totalling 21 inches and deep 
enough to penetrate the coating and expose the steel substrate. 
The control along with Samples 1 thru 3, were exposed to inland conditions 
for 13 months and the percentage of the 21 inches of scribe damage which 
showed red rusting of the steel was measured. The control showed 21 
percent rusting, Sample 1 also showed 21 percent rusting, while Sample 2 
showed 2 percent rusting and Sample 3 showed 7% rusting. 
It was also observed that during immersion the indium deposited on the 
surface of the parts as metallic indium while no indium precipitated as 
indium hydroxide.