Vapor-phase corrosion inhibitors and method of preparing same

Details Vapor-phase corrosion inhibitors and method of preparing same Vapor-phase corrosion inhibitors and method of preparing same

2002-04-30

2004-06-22

Thexton, Matthew A. (Department: 1714)

Compositions

Preservative agents

Anti-corrosion

C252S389620

Reexamination Certificate

active

06752934

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to corrosion inhibitors and more particularly to combinations of substances for use as vapor-phase corrosion inhibitors (volatile corrosion inhibitors, VCI) for protecting conventional metals for use, such as iron, chromium, nickel, tin, zinc, aluminum, copper and alloys thereof, from atmospheric corrosion.
BACKGROUND OF THE INVENTION
It is already known in general that corrosion inhibitors which tend to undergo sublimation in powder form under normal conditions and can reach metal surfaces that are to be protected through the gas phase, can be used for temporary corrosion prevention on metal objects within closed spaces, e.g., in packaging or in display boxes.
These vapor-phase inhibitors (VPI) or volatile corrosion inhibitors (VCI) are usually selected according to the type of metal to be protected and are used in the form of a powder packaged in a bag of a material that is permeable for the vapor-phase inhibitors (see, for example, H. H. Uhlig,
Corrosion and Corrosion Prevention
, Akademie-Verlag Berlin, 1970, pages 247-249; K. Barton,
Protection Against Atmospheric Corrosion
; Theory and Practice, Verlag Chemie, Weinheim 1973, pages 96 ff. or I. L. Rozenfeld,
Corrosion Inhibitors
(Russian) Izt-vo Chimija, Moscow 1977, page 320 ff; A. D. Mercer,
Proceedings of the
7
th European Symposium on Corrosion Inhibitors
, Ann. Univ. Ferrara/Italy, N. S., Sez V, Suppl. No. 9 (1990), 449 pp.).
Modern packaging materials for corrosion prevention contain VCIs either in tablet form within porous foam capsules or as a fine powder inside of polymer carrier materials. For example, U.S. Pat. Nos. 3,836,077, 3,967,926, 5,332,525, 5,393,457, 4,124,549, 4,290,912, 5,209,869, Japanese Patent 4,124,549, European Patent 0,639,657 and Unexamined German Patent 3,545,473 propose several variants whereby VCIs are introduced in the form of capsules or air-permeable plastic films, either by incorporation into cavities created by cutting open a foam and subsequently covering same with a gas-permeable material or by adding the VCI to the polymer melt intended for melt extrusion or blow molding, thus resulting in a packaging material (film or hard material) out of which the VCI components are able to sublime continuously because of the structurally induced porosity.
There have already been attempts to incorporate VCIs during foaming of polymeric solids, as described for example in Japanese Patent 58,063,732, U.S. Pat. No. 4,275,835 and German Democratic Republic Patent 295,668. In addition, packaging materials containing VCI can be produced by dissolving the VCI components in a suitable solvent and applying this solution to the respective packaging material. Methods of this type using various active ingredients and solvents are described, for example, in Japanese Patents 61,227,188, 62,063,686, 63,028,888, 63,183,182, 63,210,285, German Patent 1521900 and U.S. Pat. No. 3,887,481.
However, the VCI packaging materials produced in this way usually contain the active ingredients incorporated only loosely in the structurally induced cavities in the carrier material, whether paper, cardboard, foam, etc., so there is the danger of mechanical rupturing and escape of the active ingredient particles, so it is impossible to ensure that carrier materials pretreated in this way will still have the required specific surface concentration of VCI at the time of their use for corrosion prevention.
To eliminate this disadvantage, U.S. Pat. No. 5,958,115 describes a corrosion-inhibiting composite material which consists of a mixture of metal oxide sol, corrosion inhibitors that are capable of sublimation and additional additives and forms a firmly adhering, sufficiently porous gel film of the metal oxides and additives used on the support material, so that the corrosion inhibitors (VCIs) are released from the film at a uniform, long-lasting emission rate.
According to the ISO definition, a corrosion inhibitor is a “chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration without significantly changing the concentration of any other corrosive agent; the use of the term inhibitor should be qualified by the nature of the metal and the environment in which it is effective” (cf. Corrosion of metals and alloys—Terms and definitions, ISO 8044-1986).
The main principle in the use of VCIs is to maintain or reinforce the inherent primary oxide layer, which usually provides only limited protection but which forms very rapidly on any metal due to contact with the atmosphere, although it cannot be perceived visually without optical aids (K. Barton, loc. cit.; E. Kunze (eds.),
Corrosion and Corrosion Protection
, volume 3, Wiley-VCH, Berlin, Weinheim, New York 2001, pages 1680 ff.).
With regard to the type and properties of said primary oxide layer, the known utilitarian metals and their alloys may be divided into two categories, namely the passivatable metals, where a sufficiently strong oxidizing agent is required to maintain or recreate the protective primary oxide layer, and those metals which are classified as non-passivatable, where the passive oxide layer undergoes chemical and/or structural changes due to the action of strong oxidizing agents so that adhesion to the substrate and thus also the corrosion-preventing effect are lost.
To illustrate this distinction between the two categories of utilitarian metals, the following examples shall be used. In the ferrous materials which belong to the category of passivatable metals, the primary oxide layer consists mainly of Fe(III) oxides, for example. If the metal surface becomes moistened, which is the case when a condensed film of water develops in rooms saturated with water vapor due to a drop in temperature when a sufficiently strong oxidizing agent is not in effect at the same time, then corrosion of the metal begins by conversion of these oxides into Fe(II) compounds, e.g.:
Fe
2
O
3
+H
2
O+2H
+
+2e
−
2Fe(OH)
2
and for the anodic step of corrosion of the substrate metal:
Fe+2H
2
O→Fe(OH)
2
+2H
+
+2e
−
they function cathodically.
Metals that must be classified in the category of non-passivatable metals include, for example, copper whose primary oxide layer is sensitive to further oxidation. Its primary oxide layer is known to consist mainly of the oxide Cu
2
O and it is stable only in aqueous media which do not contain any strong oxidizing agent, regardless of pH. Under the action of oxygen in humid air, however, the oxide CuO is formed relatively rapidly and is detectable as a black deposit which cannot become intergrown with the metal substrate because of its crystal lattice dimensions (no epitaxy) and therefore cannot provide any corrosion protection. The following equation can be formulated for the starting reactions of atmospheric corrosion of copper:
Cu
2
O+H
2
O→2CuO+2H
+
+2e
−
½O
2
+2H
+
+2e
−
→H
2
O
and as the gross reaction which eliminates the passive state:
Cu
2
O+½O
2
→2CuO
Most conventional utilitarian metals are considered to be passivatable on contact with aqueous media. Thus, the case with nickel is similar to that with iron because its primary oxide layer contains Ni
2
O
3
. In the case of chromium, the passive state is caused by Cr
2
O
3
/CrOOH, and in the case of tin it is caused by SnO/SnO
2
, in the case of zinc it is caused by ZnO and in the case of aluminum by Al
2
O
3
/AlOOH. These passive oxide layers are usually maintained in neutral aqueous media or they form again spontaneously after local mechanical abrasion (abrasion, erosion) when the action of a sufficiently strong oxidizing agent is guaranteed (E. Kunze, loc. cit.).
Nitrites as salts of nitrous acid have already proven very successful as passivating oxidizing agents of this type. Therefore, they have long been used as vapor-phase inhibitors. The relatively volatile dicyclohexylammonium nitrite has already been in use as a vapor-phase inhibitor

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