Vapor-phase corrosion inhibitors and methods for their...

Details Vapor-phase corrosion inhibitors and methods for their... Vapor-phase corrosion inhibitors and methods for their...

1999-07-21

2003-04-01

McKane, Elizabeth (Department: 1744)

Chemical apparatus and process disinfecting, deodorizing, preser

Process disinfecting, preserving, deodorizing, or sterilizing

Maintaining environment nondestructive to metal

C422S010000, C252S390000, C252S391000, C252S393000

Reexamination Certificate

active

06540959

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to combinations of substances that can be used as vapor-phase corrosion inhibitors (volatile corrosion inhibitors, VCIs) in particular to protect non-ferrous metals or metals that cannot be passivated, like copper, silver, manganese, magnesium and their alloys, against atmospheric corrosion.
BACKGROUND
It is already generally known that corrosion inhibitors which in powder form tend to sublimate under normal conditions, and through which vapor phases can penetrate to metal surfaces that are to be protected, are used for temporary corrosion protection of metal objects in closed areas, eg in packaging or showcases.
As a rule these vapor-phase inhibitors (VPIs) or volatile corrosion inhibitors (VCIs) are selected according to the type of metal to be protected and used as powder, packed in bags of a material that is permeable to the vaporous VPIs (cf H. H. Uhlig, Corrosion and Corrosion Protection (German), Akademie-Verlag Berlin, 1970, pp 247-249; K. Barton, Protection against Atmospheric Corrosion—Theory and Practice (German), Verlag Chemie, Weinheim, 1973, p 96 ff; I. L. Rozenfeld, Corrosion Inhibitors (Russian), Izt-vo Chimija, Moscow, 1977, p 320 ff).
Modern means of packaging for corrosion protection contain VCIs either in pellet form in porous foam plastic capsules or as fine powder inside polymeric carrier materials. Thus, in the patents 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, JP 4,124,549, EP 0.639.657 and DE-OS 3.545.473, different variants are suggested for incorporating VCIs in pellets or plastic films permeable to air, either by enclosing them in cavities created by opening up foam plastic and then covering them by a material permeable to gas or by adding the VCIs to the polymer melt intended for blow extrusion or injection molding so that a packaging material (film or hard substance) results from which the VCI components can continuously sublimate because of the structural porosity.
Attempts have already been made to work in the VCIs during the expansion of polymer solids, as described for example in JP 58.063.732, U.S. Pat. No. 4,275,835 and DD 295.668. Furthermore, packaging containing VCIs can be produced by dissolving the VCI components in a suitable solvent and depositing them on the particular packaging material. Methods of this kind with different active components and solvents are described for example in JP 61.227.188, JP 62.063.686, JP 63.028.888, JP 63.183.182, JP 63.210.285, DE-PO 1521900 and U.S. Pat. No. 3,887,481.
But, given the fact that the VCI packaging material produced in this way usually contains the active components only loosely in the structural cavities of the carrier material, paper, cardboard and foam plastic, etc, there is danger of mechanical spreading and spilling of the active particles so that it is not possible to ensure that the pretreated carrier materials at all possess the necessary specific surface concentration of VCIs when they are used for corrosion protection.
To eliminate this drawback, DE-PS 19708285 describes a corrosion-inhibiting composite material that consists of a mixture of a metallic oxide sol, the corrosion inhibitors capable of sublimation and further additives and forms a firmly adhering, sufficiently porous gel film of the used metal oxides and additives on the carrier material from which the corrosion inhibitors (VCIs) are released at a steady, long lasting rate of emission.
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” (see “Corrosion of metals and alloys—Terms and definitions”, ISO 8044, 1986).
The major principle of using VCIs is to maintain or reinforce the inherent primary oxide layer, usually offering only limited protection, that forms very fast on every metal through contact with the atmosphere but cannot be perceived without optical aids (K. Barton, loc. cit.).
As regards the nature and properties of the mentioned primary oxide layer, the familiar commodity metals and their alloys can be divided into two categories, those where a sufficiently strong oxidizer is needed to maintain the protective primary oxide layer, and those where the passive oxide layer undergoes such chemical and/or structural changes through the action of an oxidizer that adhesion to the substrate and thus also the protective effect against corrosion are lost.
Among iron materials the primary oxide layer consists for the most part of an Fe (III) oxide. If the metal surface becomes damp, as is the case, for example, when a water film condenses, without the simultaneous action of a sufficiently strong oxidizer, then corrosion of the metal commences through transformation of these oxides into Fe (II) compounds, eg:
Fe
2
O
3
+H
2
O+2H
+
+2e
−
→2Fe(OH)
2
and where, for the anodic corrosion of the substrate metal:
Fe+2H
2
O→Fe(OH)
2
+2H
+
2e
−
To avoid this, the action of a sufficiently strong oxidizer is necessary. Nitrites and, in particular, the relatively readily volatile dicyclohexyl ammonium nitrite have consequently been used for more than 50 years as vapor-phase inhibitors (cf Uhlig, Barton, Rozenfeld, loc. cit.) and are named as a constituent of VCI compositions in numerous patents (eg U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840, 4,290,912, 4,973,448, JP 02085380, JP 62109987, JP 63210285 A, DE-PS 4040586).
The metals whose primary oxide layer is sensitive to further oxidation include e.g. copper, silver and manganese.
In Cu and Cu base materials the primary oxide film consists mainly of the oxide Cu
2
O for example. This film is only stable in hydrous media free of oxidizers, independently of the pH value. Exposed to the effect of oxygen, the oxide CuO is produced relatively fast, which is perceivable as a black deposit that, because of its crystal lattice dimensions, cannot intergrow with the metal substrate (no epitaxy) and therefore does not guard against corrosion. The initiating reactions of atmospheric corrosion can be formulated as follows:
For the creation of VCI packaging means that cannot only be used for a certain kind of metal but that are also multivalent, it was attempted to formulate VCI combinations that contain not only amine nitrites but also components which are able to protect heterogeneous cast materials and precisely those metals like copper and silver base materials against corrosion.
In the course of this it was proposed to combine nitrites with further substances capable of sublimation, like the salts of medium-strength to weak, saturated or unsaturated carboxylic acids (cf U.S. Pat. No. 2,419,327, 2,432,839, 2,432,840, DE 814.725). Although this produces protection of the common Al, Sn and Zn materials, corrosion of Cu and Mg materials in the same packaging is further promoted.
The cause of this is found in the existence of nitrite, which cannot only oxidize the primary oxide layer of the copper but reduces to ammonia NH
3
when acting as an oxidizer. This NH
3
can, on the one hand, transform the oxidic passive layer of the copper metals into soluble complexes and, on the other, create such high alkalization on Mg surfaces when humidified that a soluble magnesium hydroxo complex is produced from the existing MgO film. In both cases this means a loss of the passive state and consequently the beginning of corrosion (cf A. D. Mercer, Proc. of the 7th Europ. Symp. on Corrosion Inhibitors, Ann. Univ. Ferrara/Italy, N. S., Sez. V, Suppl. N. 9 (1990), 449 pp).
To eliminate this drawback, VCI systems were proposed that are to be suitable for corrosion protection of any metal combinations but are to make do without nitrite and amines by being composed solely of combinations of organic carboxylic acids and their salts, as for

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