Corrosion resistance deposits

Hard chromium plating provides excellent resistance to atmospheric oxidation both at normal temperatures and at temperatures of up to 650°C. It is unattacked by many chemicals, owing to its passivity. When attack takes place, this usually commences at cracks in the chromium network hence the most corrosion-resistant deposits must have a very fine structure, such as is obtained from relatively high solution temperatures using low current densities. 

Electroless nickel coating is used because of its wear and corrosion resistance. Deposition rates (10 pm/h, or 0.4 mil/h) are relatively slow compared to electrode-position methods, but the electroless depositions are more uniform than those of electroplating. Normal thickness is... 

An overdosage of brighteners may result in a brittle, stressed, less corrosion-resistant deposit that may easily peel off the base metal. 

The Fe, Co, and Ni deposits are extremely fine grained at high current density and pH. Electroless nickel, cobalt, and nickel—cobalt alloy plating from fluoroborate-containing baths yields a deposit of superior corrosion resistance, low stress, and excellent hardenabiUty (114). Lead is plated alone or ia combination with tin, iadium, and antimony (115). Sound iasulators are made as lead—plastic laminates by electrolyticaHy coating Pb from a fluoroborate bath to 0.5 mm on a copper-coated nylon or polypropylene film (116) (see Insulation, acoustic). Steel plates can be simultaneously electrocoated with lead and poly(tetrafluoroethylene) (117). Solder is plated ia solutioas containing Pb(Bp4)2 and Sn(Bp4)2 thus the lustrous solder-plated object is coated with a Pb—Sn alloy (118). 

Nickel-based aUoys have superior corrosion resistance to Hon-based aUoys. The only aUoys recommended for hot hydrochloric acid use are Ni—Mo aUoys containing 60—70% Ni and 25—33% Mo. Chlorimet (63 Ni, 32 Mo, 3 Fe) and HasteUoy (60 Ni, 28 Mo, 6 Fe) are found to be stable at aU acid concentrations in the absence of aH and Hon chlorides. Electroless nickel, a Ni—P aUoy containing 2—10% P, shows exceUent resistance to hot hydrogen chloride (71). The corrosion resistance increases with phosphoms content. This coating can be deposited on cast Hon, wrought Hon, mild steel, stainless steels, brass, bron2e, and aluminum (qv). 

Last but not least the corrosion resistance of this type of material is very poor. A partiy oxidized medium is the solution therefore during deposition oxygen was added. The final composition of Co—Ni—O also gave the right magnetic medium properties. The is strongly influenced by the O2 value. 

Decorative chromium plating, 0.2—0.5 ]lni deposit thickness, is widely used for automobile body parts, appHances, plumbing fixtures, and many other products. It is customarily appHed over a nonferrous base in the plating of steel plates. To obtain the necessary corrosion resistance, the nature of the undercoat and the porosity and stresses of the chromium are all carefliUy controlled. Thus microcracked, microporous, crack-free, or conventional chromium may be plated over duplex and triplex nickel undercoats. 

Zinc—Nickel. Steel has the best salt spray resistance when the nickel is 12—13% of the alloy. At increasing nickel contents, the deposit becomes more difficult to chromate and more noble, eventually becoming cathodic to steel. At those levels and above, corrosion resistance usually decreases and is dependent on a complete lack of porosity for protection of the steel. In efforts to replace cadmium and nickel—ca dmium diffused coatings in the aircraft industry, 2inc—nickel has insufficient wear properties for some appHcation, but is under study as an undercoat to various electroless nickel top coats (153). 

Another area where probes can be used effectively is in monitoring of deposits such as scale. One method of measurement is detecting specific ions that contribute to scale buildup or fouling another is measuring the ac tual layers. Scale and fouling often devastate the corrosion resistance of a system, leading to a costly corrosion-related plant shutdown. 

Sometimes there is no practical, economical way to reduce deposition, protect surfaces cathodically, change design and operation, or treat existing systems chemically. A material change is required. The most economical solution usually is to coat existing structures with water-impermeable, sacrificial, or corrosion-resistant materials. 

Substituting one alloy for another may be the only viable solution to a specific corrosion problem. However, caution should be exercised this is especially true in a cooling water environment containing deposits. Concentration cell corrosion is insidious. Corrosion-resistant materials in oxidizing environments such as stainless steels can be severely pitted when surfaces are shielded by deposits. Each deposit is unique, and nature can be perverse. Thus, replacement materials ideally should be tested in the specific service environment before substitution is accepted. 

---