Corrosion vacuum evaporation

Compared with other methods, vacuum evaporation produces coatings that have a most satisfactory corrosion resistance, e.g. 0.005 mm of evaporated Cd gives a degree of protection to steel similar to that afforded by 0.01 mm of electrodeposited Cd. Cadmium coatings on ferrous and other substrates can meet authoritative specifications concerning corrosion... 

To test the corrosion protection conferred by LMP - produced films, glass microscope slides bearing 5000 A - thick layers of aluminium (by vacuum evaporation) were overcoated with P-PHMDSO films. In this experimental series plasma deposits were maintained at thicknesses near 1000 A, and were produced at T ranging from 100 C to about 300 C. Plasma-coated and control samples were placed in a bath of alkaline cleaning fluid (pH 8.5) and Inspected periodically for loss of Al, as described in an earlier publication (5). 

The technique may be said to combine the advantages of vacuum evaporation and sputtering, so that excellent qualities of adhesion are obtained without a limitation of maximum thickness of the coating—while at the same time the rate of deposition can be comparatively high. Many metals, alloys, and compounds may be deposited, on both metallic and non-metallic articles. However, its use at present is mainly for functional and protective applications, particularly where high resistance to corrosion is required. Thus, as examples, aluminium may be deposited on various types of steel and on titanium for uses in the aerospace and defence industries—and can be regarded as a less hazardous replacement for cadmium electroplating. 

Concentration of acid The phosphoric acid produced in several of the above-mentioned processes may, depending upon the application (currently mainly fertilizer manufacture), have to be concentrated. Fertilizer production requires acids containing between 40 and 54% diphos-phorus(V) oxide. For transportation purposes further concentration to 52 to 72% diphosphorus(V) oxide is required. The use of certain evaporation processes, e.g. submerged burner, vacuum evaporation etc., to concentrate the phosphoric acid is problematical due to the high corrosiveness of the acid, the formation of precipitates and the release of acid-containing gases (fluorine compounds and phosphoric acid mist). Furthermore, they are very energy intensive. 

Micro-mirror. A method of measuring the corrosivity of an atmosphere that was developed at Sandia National Laboratories involves measuring the reflectivity of an optically thin metal mirror. A thin layer of metal (i.e., from 10 to 30 nm) is applied to the end of a fiber by thermal or vacuum evaporation to form a micro-mirror. A schematic of a micro-mirror system is shown in Fig. 6.34. Light passes through the optical fiber to the metal at the end of the fiber and is partially reflected. The main signal output is either the ratio of the intensity of the reflected light to that of the incident light or the ratio of the reflectivity to the initial reflectivity with a clean micro-mirror. Species from the atmosphere that chemisorb and/or react with the metal reduce the reflectivity. 

Lacquer coating (decorative coating) The topcoat that is used to give abrasion and corrosion resistance, color, and texture to a decorative coating system. The lacquer is typically applied over a reflective aluminum film deposited by vacuum evaporation and may be deposited on a flow-coated basecoat, which creates a smooth surface. See also Basecoat Topcoat. 

Vermilyea, who has defined corrosion as a process in which atoms or molecules are removed one at a time, considers that evaporation of a metal into vacuum should come within the scope of the term, since atomically it is similar to other corrosion processes. ... 

Despite these qualifications copper and its alloys are used extensively and successfully in much chemical equipment. Uses include condensers and evaporators, pipelines, pumps, fans, vacuum pans, fractionating columns, etc. Tin-bronzes, aluminium-bronzes and silicon-bronzes are used in some circumstances because they present better corrosion resistance than copper or brasses. 

Corrosive thionyl chloride may destroy the rubber vacuum seals of a rotary evaporator. Thionyl chloride can also be removed by vacuum distillation. 

---