Chromate treatments types

This type of corrosion can take place on any new surface of zinc and is best prevented by storing the metal in a dry, airy place until a protective layer has been formed. Zinc which has been properly aged in this way is safe against white-rust formation. Various methods are employed to prevent white rust. A chromate treatment is widely used for zinc-plated articles and for galvanised sheet, and occasionally for zinc die castings. Fatty substances, such as oils or lanolin, are sometimes used to protect larger items. 

Chromate treatments can be applied to a wide range of industrial metals. They are of two broad types (a) those which are complete in themselves and deposit substantial chromate films on the bare metal and (b) those which are used to seal or supplement protective coatings of other types, e.g. oxide and phosphate coatings. Types of treatment for various metals are summarised in Table 15.16. 

Typos of Chromate Treatments. Hie four types in use are alkaline oxide, chromium phosphate, chromate, and no-rinse. 

BS 6536 1985 refers to four grades of carbon steel strip coated with Al-Si alloys containing 5-11 wt.% Si. The availability of the steel grades allows account to be taken of the type and degree of forming which is to be applied to the coated steel. Various coating masses are supplied in the range 40-180 g/m which may be additionally protected by a chromate passivation treatment. 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

While minimizing blowdown volumes can be quite effective in reducing waste, removing the hazardous metals component of the waste stream can have a more dramatic effect on reducing the impact of the blowdown on the environment. Chromate based corrosion inhibitors have historically been the mainstay of cooling water treatment systems. But in recent years, some plants have elected to use nonchromate inhibitors in order to reduce the potential environmental problems associated with chromates. Types of nonchromate inhibitors that have proved useful and that have in some situations performed as well as chromates include (Roti 1985) ... 

NOTE The consumption of chemical treatment in a cooling system is directly proportional to total water consumption (for any given operating conditions, such as cycles of concentration), but the selection of a suitable type of chemical program (e.g., chromate, alkaline zinc) is based on a combination of factors, including system metallurgy, operating conditions, anticipated or actual... 

Two particular types of end treatments have enhanced the usability of lead chromates and lead molybdates in plastics applications. So-called predarkened lead chromates have been surface treated with antimony to improve outdoor weather-ability. A more significant end treatment has been encapsulation of both chrome... 

Figure 10.8 Scanned image of the surface of two alloy panels showing adhesion failure caused by the omission of O2 plasma treatment of the substrate prior to plasma film deposition and application of the primer (Deft 44-GN-72 MIL-P-85582 Type I Waterbased Chromated Control Primer), a) Panel after Skydrol LD4 fluid resistance test, which had the O2 plasma treatment prior to film deposition and primer application, b) Panel after scribed wet (24-h immersion in tap water) tape test, which had not been treated with the O2 plasma treatment prior to film deposition and primer application.

Chemical Treatment. A wide variety of chemicals and water treatments are used for corrosion control. Corrosion inhibitors usually act by forming some type of impervious layer on the metallic surface of either the anode or cathode that impedes the reaction at the electrode and thereby slows or inhibits the corrosion reaction. For example, various alkali metal hydroxides, carbonates, silicates, borates, phosphates, chromates, and nitrites promote the formation of a stable surface oxide on metals. The presence of these chemicals in the electrolyte allows any faults in the metal surface or its oxide film to be repaired. If they are used in too small a quantity as anodic inhibitors, they may promote intense local attack because they can leave a small unprotected area on the anode where the current density will be very high. This is particularly true of chromates and polyphosphates. 

Fe-, Cd-, and Sn-based alloys. Chromate solutions are also used to seal coatings formed by anodizing or hydrothermal treatment. CCCs are primarily used to improve adherence of subsequently applied organic coatings, or to impart corrosion resistance under atmospheric exposure conditions. Certain types of CCCs contain labile hexavalent chromium (Cr(VI)), which can be leached from the coating when it is contacted by an attacking solution. This Cr(VI) can be transported... 

The most important treatment is the conversion coating (see Chapter 5.3 of the same volume). This type of treatment is typically used for zinc or cadmium layers or on bulk metals like aluminum or magnesium. The classical conversion coating is chromating, the formation of a metal oxide/chromium oxide film. We will discuss the process for the example of zinc layers. 

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