Plasma film corrosion protection

This, combined with the feasible scale-up of microwave plasma apparatus holds out the promise of larger-scale applications for plasma produced films in the corrosion protection area. 

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). 

As stated, the capability of plasma deposits to reduce the access of water to corrosion-sensitive surfaces may be an important motivation for their application in corrosion protection. In order to study this property, Kapton polyimide film was selected as the substrate because of its high inherent permeability to water and its ability to resist elevated temperatures. The response of Kapton film overcoated by PPHMDSO to the permeation of water vapor is shown in Fig. 1. Clearly, the presence of the organo-silicone plasma film greatly reduces water permeation. The magnitude of the effect is much enhanced when plasma polymers are produced at high T and p. 

Plasma surface treatment of many polymers, including fabrics, plastics, and composites, often occurs. The production of ultra-thin films via plasma deposition is important in microelectronics, biomaterials, corrosion protection, permeation control, and for adhesion control. Plasma coatings are often on the order of 1 100 nm thick. 

XPS Cls/Si2p ratio steadily increases with the reaction time (film thickness) by a closed-system plasma polymerization, while the ratio more or less stays at a constant level by a flow system cathodic polymerization, as shown in Figure 4.10. Such a graded ultrathin film was found to provide an excellent corrosion protection of aluminum alloy when an organic coating was applied on top of the ultrathin film [8]. 

Although the panel with the plasma deposited film followed by priming with E-coat is visually better, the use of the corrosion width provides a method for quantifying the improvement in the corrosion performance. Also the factor of about 2 difference in corrosion width between the two chromate conversion-coated panels is difficult to obtain from the qualitative difference observed from the scanned images. It can be seen from this comparison of three panels that the use of the measured corrosion width makes the differentiation of corrosion performance much easier. This method of evaluating corrosion test results is used to determine if the combination of the two bests could indeed yield the better corrosion protection of aluminum alloys. 

Thus, without SAIE, Parylene C film, which has excellent barrier and physical properties, cannot be utilized in corrosion protection of a metal. Conversely, SAIE is the key to yield an excellent corrosion protection systems. It is also important to recognize how a nanofilm of hydrophobic amorphous network of plasma coating can prevent the initiation of the salt intrusion process. 

Parylene N to smooth surface materials has been reported with the application of plasma depositions [13,14]. It was reported that excellent adhesion of Parylene C coating to a cold-rolled steel surface was achieved using plasma polymer coatings, in turn giving rise to corrosion protection of the metal [15]. Another major deficiency of Parylene C is its poor painting properties when paint is applied on a Parylene C film, due to its extremely hydrophobic surface. Because of this, surface modification of Parylene films is necessary to enhance their adhesion performance with spray primers. 

While a copper-enriched surface has the implication of always causing accelerated electrochemical corrosion, replacing the native, hydroxylated, mixed Al-Mg oxide layer with a thin stable oxide layer seems to allow the plasma films to tightly adhere to the alloy surface. This adhesion, coupled with the barrier properties of the films, appears to provide additional protection of the oxide layer from contact with corrosive agents. 

It is clear from the study on pure iron that oxides participate in LCVD of TMS, and characteristics of plasma polymer films differ depending on the extent of oxides present on the surface when LCVD is applied. Oxides on the surface of pure iron are more stable than those on steel and hence more difficult to remove, but this can be effected by plasma pretreatment with (Ar + H2) mixture. SAIL by LCVD involving removal of oxides provides excellent corrosion protection of pure iron. The key factor of SAIL by LCVD for corrosion protection of metals in general is the handling of oxides, which depends on the characteristic nature of the metal oxide to be handled. Once strong chemical bonds were formed between nanofilm of plasma polymer, either through oxides or direct bonding to the substrate metal, the LCVD film acts as the barrier to corrosive species. 

In Fig. 3, a 30-nm nanoparticle of cobalt is shown, which had been coated with a 5-nm plasma film, also of PP-CeFw, for protection against corrosion by atmospheric moisture. 

Studies in the 1990s revealed the good corrosion protection properties of silicon-based plasma polymers on steel substrates and the cmcial influence of the pretreatment process on the stability of the resulting interface [92-101]. The pretreatments for trimethylsilane-based films may consist of an oxidative step (02-plasma) to remove organic contaminations from the substrate and a second reductive step (Ar/H2-plasma) to remove the metal oxide layer. Although the successive application of both steps provides the best corrosion protection of various plasma treatments for steel in combination with a cathodic electrocoat, little is known about the chemical structure of the interface. Yasuda et al. [101] and van Ooij and Conners [97] in particular have shown that the deposition of plasma polymers on steel and galvanized steel might even substitute the chromatation process. 

Concerning anti-corrosion properties, Tafel curves allow the calculation that the corrosion current for this kind of coating is slightly below that for classical Ce-based ppHMDSO films. Images of HMDSO treated samples after 25 days in a salt spray chamber are shown in Fig. 12.11(c,d) which compare the resistance of HMDSO plasma treatments with and without ethanol. An increase in corrosion protection is afforded by the presence of solvent, since pitting corrosion is reduced. This last result corroborates those obtained by electrochemistry. 

The establishment of clean and efHcient atmospheric plasma technologies to replace traditional methods to clean, create depositions of thin films, and functionalize surfaces of metal substrates constitutes a very critical area of current research and development. The increasing concern for the development of environmentally friendly and sustainable technologies has led to a focus on cold plasma technology, which represents an efficient alternative. Atmospheric plasma treatments can effect the removal of oil from aluminum surfaces, for example. This surface conditioning can also act as a preparation step for thin Him deposition of monomers such as hexamethyldisiloxane to achieve corrosion protection. This interfacial layer can also be functionalized to favor the adhesion of additional polymer layers. 

This plasma polymerization process has been used to deposit hybrid films from a variety of organosilane precursors with applications ranging from corrosion protection [223-225], biomedical films for implants [226], barrier coatings for packaging [227-229] to dielectric layers in integrated electronic circuits [230]. 

Overcoats are an integral part of thin-film disk structures. Their primary role is to provide wear protection. The most common overlayers, such as Rh, plasma-polymerized coatings, SiOz, and carbon, are all chemically stable if they were fully to cover the disk surface, they would provide good corrosion resistance. The thinness of the overcoats and the roughness of the surface preclude perfect coverage and open up the path for localized corrosion at the sites where the magnetic layer is exposed to the environment. 

Corrosion. Aluminum is a not a noble metal and is attacked by both alkali and acidic solutions. Because of the presence of a surface A1203 film, the metal is protected against corrosion [Diggle et al.136, Borgmann et al.137]. This oxide film, however, is easily penetrated, for instance, by the presence of chlorine ions which remain in the resist after a chlorine based plasma etch. Also, the presence of Cu in the aluminum weakens the corrosion resistance of the alloy by the presence of an unfavorable electrochemical couple (A1/Cu2+). 

APD, or cathodic arc deposition, has been traditionally applied for preparing dense thin films of corrosion-resistant, protective, and decorative coatings, such as TiN, TiC, and CrN [25-27], This method is usually regarded as physical vapor deposition (PVD), but it differs from other PVD methods in that energetic plasma ions condense to form dense films in APD, while in other PVD methods, it is the vapor of neutral atoms that condense [25],... 

In conclusion, plasma polymerization provided the possibility to deposit carefully designed, highly adherent and pinhole-free thin polymer films onto various substrates, and thus to control adhesion between various types of solid surface. Moreover, plasma-deposited polymer films can be used to protect metals and other substrates from environmental attacks, for instance by corrosive agents. Further information on these topics is available in Ref. [80]. 

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