Phosphoric anodizing treatment

This phosphoric anodizing treatment is most favoured in the US. It has an additional advimtage on the large scale that the spent solution is more easily disposed of than one containing chromium. However, the absence of chromium at any stage of the treatment renders the use of a chromate primer essential. The favoured material is strontium chromate. 

In this work, the efTect of anodic oxidation treatments on activated carbon fibets (ACFs) was studied in the context of Cr(Vl), Cu( II), and Ni( II) ion adsorption behaviors. Ten wt% phosphoric acid and sodium hydroxide were used for acidic and basic electrolytes, respectively. Surf properties of the ACFs were determined by XPS. The specific surface area and the pore stnicture were evaluated from nitrogen adsorption data at 77 K. The heavy metal adsorption rates of ACFs were measured by using a UV spectrometer and 1C P. As a result, the anodic treatments led to an increase in the amount of total acidity by an increase of acidic functional groups such as carboxyl, lactone, and phenol, in spite of a decrease in specific surface area, due to the pore blocking by increased acidic functional groups. 

In contrast to the relatively open cellular structures of etched and phosphoric acid-anodized aluminum, the chromic acid anodize treatment produces a very thick, dense oxide layer, consisting of solid columns, represented by the drawing in Figure 14. This presents a relatively smooth surface with no protrusions. 

Figure S4. SSIMS spectra from oxide nims formed on aluminum after anodization treatment in phosphoric acid for 3 or 5 s 1186) A) Positive SSIMS spectrum, 3 s B) Positive SSIMS spectnim. 5 s C) Negative SSIMS spectrum. 3 s D) Negative SSIMS spectrum. 5 s...

Zhang J.S., Zhao X.H., Zuo Y. and Xiong J.R (2008), The bond strength and corrosion resistance of aluminium by anodizing treatment in phosphoric acid modified boric/ sulfuric acid bath , Surf. Coat Technol., 202, 3149-56. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

It is possible to obtain conditions in which the anodic him continues to grow to form a blue or black layer, and this, although not exceptionally protective, has uses in the treatment of baking pans. A typical anodising solution contains 100g/1 Na2HP04 IZHjO and 20 ml/1 phosphoric acid, and is used at 350 A/m at 60-90°C for about 10 min. 

Tests for quality of sealing of anodic coatings have become internationally standardised. They include dye spot tests with prior acid treatment of the surface (ISO 2143 1981 and BS 6161 Part 5 1982), measurement of admittance or impedance (ISO 2931 1983 and BS 6161 Part 6 1984), or measurement of weight loss after acid immersion (ISO 3210 1983 and BS 6161 Part 3 1984, and ISO 2932 1981 and BS 6161 Part 4 1981). Of these the chromic-phosphoric acid immersion test (ISO 3210) has become the generally accepted reference test. 

Various pre-treatment protocols have been developed including pickling and anodic/cathodic pulses to remove the oxide films. It was apparent that different types of steel require different pre-treatments, i.e. cast pieces behave differently to rolled pieces. Significant success was achieved in electropolishing cast pieces and the finish obtained with the ionic liquid was superior to that with phosphoric add, however, the converse was true for rolled pieces because the oxide film is thicker in the latter samples and hence slower to dissolve in the ionic liquid. 

One very beneficial chemical pretreatment treatment for aluminum substrates is phosphoric acid anodization (PAA), which provides an oxide coating that is inherently hydration-resistant. Its stability is due to a layer of phosphate incorporated into the outer A1203 surface during anodization. 

The formation of defects in porous anodic alumina films fabricated in phosphoric acid solutions has been studied. TEM and SEM examinations revealed that defects appear at the triple point junctions of the cell boundaries, where neighbouring celis meet. The defects represent voids in the anodic alumina and develop into spatial structures that comprise a central void at the triple-point junction extending into tubular branches that pass toward the pore wall. The defects are generated continuously during the anodic film growth under appropriate anodising conditions. Additional post-anodising treatment also results in periodic arrays of circular nanoholes in the pore walls. 

The PPQ adhesive system made up of PPQ adhesive resin, dilute PPQ resin primer, and chromic-acid anodized surface treatment was selected based upon the data generated from this study, as well as from internal Boeing research programs. Lap shear data in Fig. 2 shows the phosphoric-acid anodized surface treatment superior to the chromic-acid anodized. However, Boeing data on both these surface treatments have shown that phosphoric-acid anodize is not stable in long-term (125 hr) elevated temperature (600 F) environments. Chromic-acid anodize exhibited good thermal... 

In contrast to the above slow batch methods, A.C. anodizing processes have been developed for the treatment of aluminium at the coil stage, using either sulphuric acid or phosphoric acid electrolytes. 

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