Pretreatment for metals

Metal and oxide surfaces. In joints involving metallie substrates, the adhesive stieks to the metal surface oxide layer and not to the metal itself. Sueh joints ean eause problems in service because oxide structures, and bonds to them, are susceptible to environmental attack. 

Chemical treatment of ferrous alloys with sulphuric or hydrofluoric acid, is not straightforward because of the precipitation of free carbon on the surface known as smutting , and the consequent need for de-smutting immediately after etching(45). The metal then needs to be rinsed and washed in running water before transference 

Galvanised surfaces are characterised by relatively weak zinc oxide which may need to be removed by mechanical or, preferably, chemical means a number of electrodeposition(35) and etch processes are described(47). Lees(48) cautions that although successful pretreatment is possible, the zinc layer itself remains as a potential source of weakness, since it might be stripped off by a good adhesive. Adams and Wake(45) further note that certain adhesives and anhydride-cured epoxies may form soaps at the zinc-adhesive interface. Nevertheless, for only moderately demanding situations 

Some comparisons of the effect of surface pretreatment on mechanical joint strength, or measurable adhesion, are reproduced in Tables 3.5-3.7. It is stressed again that the important consideration is the effect on long-term bond integrity, and not on short-term strength. 

Surface pretreatment Substrate Martensitic Austenitic stainless stainless Mild steel steel steel Mean bond strength (MN m--)  

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Recently, many research efforts have been directed at developing pretreatments for metal surfaces which produce oxide layers with pores, fibrous projections, or microscopic roughness which can enhance metal/polymer adhesion by mechanical means. In order for the pretreatments to lead to an increase in durability, the oxide layers formed must be stable under environmental conditions. The bulk 3l-33 S2-128> of the research in this area has been completed in an attempt to increase the durability of... 

Rubio, R. and Ure, A.M. (1993) Approaches to sampling and pretreatment for metal speciation in soils and sediments. Int.f. Environ. Anal. Chem., 51, 205-217. 

Cleaning metals will remove oily soils but will generally not remove rust and corrosion from substrates to be coated. Abrasive cleaning wiU remove corrosion products, and for this reason it is also considered a pretreatment, because the impingement of blasting media and the action of abrasive pads and brushes roughen the substrate and therefore enhance adhesion. The other pretreatments use aqueous chemical solutions, which are apphed by immersion or spray techniques. Pretreatments for metallic substrates used on industrial products are discussed in this section. Because they provide corrosion protection to fer-... 

Prior to analytical use, steps of pretreatment will depend on the material and experimental conditions. Pretreatment for metal electrodes are usually made by polishing or by potential cycling.Carbon based electrodes need ehemieal/electrochemical treatments, or even thermal activation [16]. 

Chemical Oxidation. Chemical oxidation can be appHed ia iadustrial wastewater pretreatment for reduction of toxicity, to oxidize metal complexes to enhance heavy metals removal from wastewaters, or as a posttreatment for toxicity reduction or priority pollutant removal. 

Eor the cover-coat direct-on process, a ferric sulfate [10028-22-5] Ee2(S0 2> etch is included in the metal pretreatment for rapid metal removal. It is designed to remove ca 20 g/m (2 g/ft ) of iron from the sheet metal surface. Hydrogen peroxide [7722-84-1/, H2O2, is added intermittently to a 1% ferric sulfate solution to reoxidize ferrous sulfate [7720-78-7] EeSO, to ferric sulfate. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Fe electrodes with electrochemically polished (cathodically pretreated for 1 hr) and renewed surfaces have been investigated in H20 + KF and H20 + Na2S04 by Rybalka et al.721,m by impedance. A diffuse-layer minimum was observed at E = -0.94 V (SCE) in a dilute solution of Na2S04 (Table 19). In dilute KC1 solutions E,njn was shifted 40 to 60 mV toward more negative potentials. The adsorbability of organic compounds (1-pentanol, 1-hexanol, cyclohexanol, diphenylamine) at the Fe electrode was very small, which has been explained in terms of the higher hydro-philicity of Fe compared with Hg and Hg-like metals. 

Barometric chemisorption. Chemisorption on catalysts is measured routinely by the barometric method. The equipment is very similar to that commonly used in texture determination by physical adsorption (see Section 3.6.2), except that for chemisorption measurements facilities for pretreatment of the samples should be present. In particular for metal catalysts often the catalyst is received in a partly or fully oxidized form and, as a consequence, reduction is required when one wants to measure the amount of active sites. Moreover, during storage adsorption of various molecules might occur and evacuation is... 

As always in chemisorption measurements, pretreatment of the samples should be done with care. For metal catalysts prepared from oxides in particular this is experimentally troublesome because a reduction step is always needed in the preparation of the metal catalyst. Hydrogen or hydrogen diluted with an inert gas is usually used for the reduction but it is difficult to remove adsorbed H2 from the surface completely. So, after reduction the metal surfaces contains (unknown) amounts of H atoms, which are strongly retained by the surface and, as a consequence, it is not easy to find reliable values for the dispersion from H2 chemisorption data. 

When HDT is a feed pretreatment for HCK, the unit could consist of one or two reactors, with or without intermediate scrubbing. The use of zeolitic-noble metal catalysts imposes careful scrubbing of ammonia and H2S prior to the corresponding HCK reactor. 

A large segment of the metal parts produced by industry are painted for both decorative purposes as well as to increase the corrosion resistance and extend the useful life of the product. To obtain maximum quality from painted metal articles, it is of paramount importance to pretreat the metal parts with a conversion coating process. ( 1,2) Pretreatment processes contribute a significant improvement in corrosion protection and durability to metal articles by ... 

There are essentially three main steps in a conversion coating process cleaning, conversion coating, and post-treating. These three different, but equally important, steps in the pretreatment of metal articles will be discussed in more detail for the purpose of providing a background for the main emphasis of this paper, the post-treatment part of the conversion coating process, and more specifically chromium-free polymeric post-treatments which have been developed in recent years to replace the environmentally unacceptable chromate systems. 

This method is used for curing coatings and inks on plastic cups, tubs, tubes or metal cans.11 The parts are placed on mandrels, which are attached to a rotating device. This device moves them through the individual stations feed, pretreat (for plastics, most frequently corona or flame), printing, curing and take-off. The printing is done by dry offset (see Section 7.5.3). 

Figure 3 241 illustrates that the durability of metal/polymer adhesion systems can greatly be influenced by the metal pretreatment chosen 1 K Therefore, it is very important to select the best pretreatment for a given system. 

A summary of some of the more common metal pretreatments is given by Derjaguin25). The literature also describes many specialized pretreatments for steel 26-29>, stainless steel 22,30), aluminum 23,31 35 copper 36 41) and other metals... 

At slightly higher temperatures (100°-200°C) catalysts consisting of chlorided alumina in combination with a noble metal, such as platinum, are used. As a cocatalyst HC1 or an organic chloride is supplied with the feedstock. The high reactivity of these catalyst systems requires careful feed pretreatment for removal of deactivating materials. Several plants (1, 2) using this type of catalyst, and one version of this process especially developed to convert C5/C6 feed, have recently been built. 

It was shown by these authors that the amount of nitrogen present during pretreatment of a catalyst affects the ultimate activity for ammonia synthesis (206). Specifically, it was found that treating H2-reduced small particles with ammonia at 670 K, followed by re-reduction of the catalyst with a H2 N2 gas mixture, gave rise to an increase in the catalytic activity compared to the activity measured after H2 reduction alone. However, when the catalyst in this high-activity state was further treated with H2 alone at 670 K, the catalytic activity was found to decrease to that value observed before the above ammonia treatment. Subsequent ammonia treatment returned the catalyst to its high-activity state. No such effects were observed for metallic-iron particles greater than 10 nm in size. 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

Far from the metal trace analysis, our initial studies with BCFMEs were focused on the determination of folic acid [122], In this case, the main goal was the optimisation of the electrode pretreatment for this analyte. An acidic medium (0.1M perchloric acid) was considered optimum for folic acid determination by differential pulse voltammetry. A linear range between 2.0 x HT8 and 1.0 x 10 6M with a detection limit of 1.0 x 10 8M was obtained. Nevertheless, in this work, the adsorptive properties of the folic acid on mercury were noted and the employment of mercury-coated carbon fibre UMEs for folic acid determination has been targeted as a future goal. 

Secondary Ion Mass Spectrometry used as a solo Instrument or in concert with other methods has proven to be an excellent technique for studying the surface chemistry of adhesive bonding materials. The application of SIMS is shown in re.lation to pretreatments of metals and alloys, chemistry and structure of adhesives, and locus of failure of debonded specimens. 

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