Alkaline degreasing/etching

The aluminium foil, which is coated in rolling oils, is first degreased/etched using an aqueous alkaline solution of Ridoline 72 (sodium hydroxide NaOH) or, better, Ridoline 305 (potassium hydroxide KOH). The use of potassium hydroxide allows a lower bath temperature to be used — generally about 55°C. 

In this procedure the first step, once again, is to pickle the aluminium substrates in accordance with the specifications quoted above. Under certain circumstances it may be sufficient to use only an alkaline degrease before anodising. The etched panels are then clamped to the anode of a standard anodising bath and are immersed in a solution of phosphoric acid at 25°C. Anodising then takes place in accordance with one or other of several European and American specifications [29,30]. A typical bath composition is ... 

The alkaline peroxide etch, dependent on bath conditions, gives an oxide thickness of 60-200 nm. The process comprises a 3-5 min alkaline degrease in Kelite 19 at 60-71°C. The washed components are then immersed for 23-27 min, at 55-65°C, in a bath containing 0.5 M sodium hydroxide and 0.4 M hydrogen peroxide (H2O2). The etched substrates are then washed and air-dried at 65°C. 

Several pretreatments are recommended by the British Standards Institute [66]. In essence, these methods cover solvent and/or alkaline degreasing followed by surface abrasion or by the use of a chemical etchant. Grit blasting, using chiU-cast iron shot, glass or alumina, is the ideal abrasion technique. Etchants based on sulphuric, hydrochloric or phosphoric acid are recommended etching conditions are 5-30 min at temperatures from ambient to 65°C. 

Uses Defoamerfor aq. or nonaq. systems, metal cleaning/degreasing, etching, cutting oils, atmospheric dyeing processes Features High alkaline resist. 

The general sequence of surface preparation for ferrous surfaces such as iron, steel, and stainless steel consists of the following methods degreasing, acid etch or alkaline clean, rinse, dry, chemical surface treatment, and priming. The chemical surface treatment step is not considered a standard procedure, but it is sometimes used when optimum quality joints are required. It consists of the formation of a corrosion-preventing film of controlled chemical composition and thickness. These films are a complex mixture of phosphates, fluorides, chromates, sulfates, nitrates, etc. The composition of the film may be the important factor that controls the strength of the bonded joint. 

Titanium is widely used in aerospace applications that require high strength-to-weight ratios at elevated temperatures. As a result, a number of different prebonding surface preparation processes have been developed for titanium. These generally follow the same sequence as for steel and other major industrial metal substrates degrease, acid-etch or alkaline-clean, rinse and dry, chemical surface treatment, rinse and dry, and finally prime or bond. Mechanical abrasion is generally not recommended for titanium surfaces. 

Medium- to high-strength bonds, suitable for noncritical applications Optimum bond strength per FPL etch process. Specified in ASTM D 2651 and MIL-A-9067. Solvent degrease may replace alkaline cleaning. 

Several surface preparations of aluminum are widely used in the aircraft industry. " The important steps in all surface preparations are vapor degrease, alkaline clean, rinse with water, chromic acid etch (FPL Etch), rinse with water, anodize with chromic acid or phosphoric acid, rinse with water, and dry at 140-160°F. Details of the procedures for the surface preparation of aluminum and other metals and composites are available from a number of sources. 

Solvent degrease Alkaline deoxidize Rinse, 5 min, RT Etch, 65 °C... 

Fig. 2.17. Strengths of some single lap joints of aluminium bonded with an epoxy-polyamide adhesive on exposure to air at 43°C and 90% relative humidity for various times. Metal surface treatments are etching in chromic-sulphuric acid, O alkaline etching, solvent degrease, and phosphoric acid etching, (after Butt and Cotter ).

The Iridite process used a standard cleaning operation (a hot alkaline etch/degrease) followed by a spray-rinse and then immersion in the mixed Iridite... 

Whichever route is being followed, the first stage is that of degreasing, by one of the methods already discussed, to clean the surfaces to be bonded. This can be followed or, occasionally, even replaced by a suitable alkaline etch or acidic emulsion cleaning [52],... 

The adherends are initially degreased using standard procedures. This process then makes use of an acid etch (to replace grit blasting) followed by an alkaline oxidation (to replace a peroxide oxidation). The details are as follows ... 

Effect of high humidity (97% r.h. at 43°C) on the strength of aluminium joints bonded with an epoxide polyamide adhesive. After Butt and Cotter (1976). Surface treatments are o chromic-sulphuric acid etch, alkaline etch (commercial formulation], solvent degrease,... 

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