Silicon corrosion

Metallization of PS by the wet chemical corrosive deposition is a subject of both scientific and applied interests [1-3]. The basic reaction of this process is substitution of silicon atoms by metal ones. Such reaction is an example of silicon corrosive dissolution under the oxidizing agent. The metals with a redox potential more negative than hydrogen may be only used as an oxidizer. 

Steinem C, Janshoff A, Lin VS-Y, Volcker HE, Ghadiri MR (2004) DNA hybridization-enhanced porous silicon corrosion mechanistic investigations and prospect for optical interferometric biosensing. Tetrahedron 60 11259-11267... 

Common alloying elements include nickel to improve low temperature mechanical properties chromium, molybdenum, and vanadium to improve elevated-temperature properties and silicon to improve properties at ordinary temperatures. Low alloy steels ate not used where corrosion is a prime factor and are usually considered separately from stainless steels. 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Silicon. Sihcon [7740-21-3] added to copper forms alloys of high strength and toughness along with improved corrosion resistance, particularly in acidic media. Sihcon in small amounts can improve fluidity. Sihcon is a very harmful impurity in leaded tin bron2e alloys, however, because it contributes to lead sweat and unsoundness. 

The engineering properties of electroless nickel have been summarhed (28). The Ni—P aHoy has good corrosion resistance, lubricity, and especiaHy high hardness. This aHoy can be heat-treated to a hardness equivalent to electrolytic hard chromium [7440-47-3] (Table 2), and the lubricity is also comparable. The wear characteristics ate extremely good, especiaHy with composites of electroless nickel and silicon carbide or fluorochloropolymers. Thus the main appHcations for electroless nickel are in replacement of hard chromium (29,30). 

Nickel increases toughness and improves low-temperature properties and corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance, and resistance to oxidation. Molybdenum provides strength at elevated temperatures. 

High-silicon cast irons have excellent corrosion resistance. Sih-con content is 13 to 16 percent. This material is known as Durion. Adding 4 percent Cr yields a product called Durichlor, which has improved resistance in the presence of oxidizing agents. These alloys are not readily machined or welded. 

Bronzes are somewhat similar to brasses in mechanical properties and to high-zinc brasses in corrosion resistance (except that bronzes are not affected by stress cracking). Aluminum and silicon bronzes are very popiilar in the process industries because they combine good strength with corrosion resistance. 

X-ray analysis of corrosion products and deposits removed from internal surfaces showed 68% iron, 12% phosphorus, 8% silicon, 3% sulfur, and 2% each of zinc, sodium, chromium, and calcium other materials made up the remainder of deposits and corrosion products. 

Copper-alloy corrosion behavior depends on the alloying elements added. Alloying copper with zinc increases corrosion rates in caustic solutions whereas nickel additions decrease corrosion rates. Silicon bronzes containing between 95% and 98% copper have corrosion rates as low as 2 mil/y (0.051 mm/y) at 140°F (60°C) in 30% caustic solutions. Figure 8.2 shows the corrosion rate in a 50% caustic soda evaporator as a function of nickel content. As is obvious, the corrosion rate falls to even lower values as nickel concentration increases. Caustic solutions attack zinc brasses at rates of 2 to 20 mil/y (0.051 to 0.51 mm/y). 

Internal surfaces were covered with a tan deposit layer up to 0.033 in. (0.084 cm) thick. The deposits were analyzed by energy-dispersive spectroscopy and were found to contain 24% calcium, 17% silicon, 16% zinc, 11% phosphorus, 7% magnesium, 2% each sodium, iron, and sulfur, 1% manganese, and 18% carbonate by weight. The porous corrosion product shown in Fig. 13.11B contained 93% copper, 3% zinc, 3% tin, and 1% iron. Traces of sulfur and aluminum were also found. Near external surfaces, up to 27% of the corrosion product was sulfur. 

Internal surfaces were covered by loosely adherent corrosion product and deposit. Much of the corrosion product was cuprous oxide. Substantial amounts of iron, silicon, aluminum, zinc, and nickel were also found. Not unexpectedly, chlorine concentrations up to 2% by weight were present sulfur concentrations of about 1% were also found. 

If Ihe main incoming male contacts are made ol aluminium alloy, which is normally a eompnsilioii of aluminium-magnesium and silicon, they must be provided wiili a coat of bron/e. copper and tin to give it an adequate mechanical hardness and resistance to corrosion. For more details refer to Section 27.2..5. 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

Another important factor is the corrosiveness of the adhesive. This may be especially important in those cases where the PSA has direct contact with the bare wire, the electronic component, or the silicon wafer in a dicing operation. In those cases where an electrical current is running through the device, electrolytic corrosion processes may occur, especially if moisture can penetrate into the adhesive or bond line. 

Silicones are being used in advanced fields of technology. One such field is aerospace where extreme environmental conditions are experienced. These applications include coatings for firewalls, windshields and other thermal barriers, and corrosion protection. In all these applications, the silicones are often not visible. 

Nickel/silicon alloy (10% silicon, 3% copper, and 87% nickel) is fabricated only as castings and is rather brittle, although it is superior to the iron/silicon alloy with respect to strength and resistance to thermal and mechanical shock. It is comparable to the iron/silicon alloy in corrosion resistance to boiling sulfuric acid solutions at concentrations above 60%. Therefore, it is chosen for this and other arduous duties where its resistance to thermal shock justifies its much higher price compared with iron/silicon alloys. 

These alloys have corrosion resistance similar to that of copper, with mechanical properties equivalent to mild steel. Because silicon bronzes do not generate sparks under shocks, they can be used in the fabrication of explosion-proof equipment. Compared to tin bronzes, the tinless bronzes have a higher shrinkage (1.7-2.5% against 1.3-1.5% of tin bronzes) and less fluid-flow, which is an important consideration in designing. 

These are used as corrosion-resistant materials. Examples are given in Table 3.32. Some can be strengthened by heat treatment. The alloys containing substantial amounts of silicon have the best foundry characteristics and a high resistance to corrosion, but are not readily machined. 

Silicon used for diffusion treatment of carbon steels enhances corrosion resistance to sulfuric acid. Such a treatment has the surface durability of iron/silicon alloys without their marked brittleness. 

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