Base metals corrosion

Incompatibilities and Reactivities Water, alcohols, bases, metals (corrosive), amihes [Note Decomposes in water to form chloroacetic acid hydrogen chloride gas.] ... 

Aune, T. K. (1983), Minimizing base metal corrosion on magnesium products. The effect of element distribution (structure) on corrosion behavior, in Pmc. 40th World Magnesium Conference, Toronto, Ontario, Canada, June, 1983, McLean, VA International Magnesium Association. 

Silica and silicate deposits cannot be removed with HCI. Where such scales are a problem, hydrofluoric acid is used either alone or in conjunction with HCI (or other acids). In the latter case, the HF is usually generated in situ by means of the addition of NH4HF2. The HCI/HF mixture has the added features of being more aggressive toward iron scales and reducing base metal corrosion by Fe in magnetite and hematite deposits, a result attributable to the formation of a strong complex between HF and Fe3+.97 pqc additional discussion, see the section on ferric ion corrosion. 

General uniform corrosion on pure magnesium has been drastically reduced by the use of inhibitors sueh as ehromate, dichromate, molybdate, nitrate, phosphates and vanadates that promote the formation of a protective layer, tend to retard corrosion (Ghali, 2006). Addition of substances that can form soluble complexes as tartrate, metaphosphate, etc. or insoluble salts as oxalate, carbonate, phosphate, fluoride, etc. is efficient at reducing corrosion. Adding soluble chromates, neutral fluorides or rare earth metal salts is effective in reducing magnesium-base metal corrosion (Schmutz et al, 2003). 

Corrosion of steel and cast iron occurs in all common environments. The rate and extent of corrosion vary from mild attack in dry, clean environments to highly accelerated attack in marine or industrial areas where corrosive fumes are present in air. Table 4 lists paints selected for service in a wide range of corrosive conditions. The rate of the base metal corrosion where paints are used should not exceed approximately 1.3 mm/yr... 

Base metal corrosion behaviour in the above formulations was studied at SOX and 90"C by weight loss method for carbon steel, monel-400, zircalloy-2 and stainless steel (304 and 403) using rectangular specimens. Corrosion on stainless steel (304 and 403), monel and zircalloy-2 was found to be negligible. In the formulations containing picolinic acid, carbon steel exhibited higher corrosion rates than EDTA based formulations. 

The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

Gold [7440-57-5] Au, is presumably the first metal known and used by humans. It occurs ia nature as a highly pure metal and is treasured because of its color, its extraordinary ductility, and its resistance to corrosion. Early uses ia medicine and dentistry date to the ancient Chinese and Egyptians. In the Middle Ages the demand for gold led to the iatense, unsuccesshil efforts of alchemists to convert base metals iato gold. These pursuits became the basis for chemical science. The search for gold has been an important factor ia world exploration and the development of world trade. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

As a final step, anodized parts must be sealed to ensure corrosion resistance of the anodic coating. Sealing involves plugging the anodic pores completely so contaminants caimot reach the base metal. A variety of sealing methods are used by anodizers (see Sealants). 

Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe O layer. Iron, in contact with the boiler water, forms magnetite and the protective layer is continuously restored. However, as long as a high caustic concentration exists, the magnetite is constantiy dissolved, causing a loss of base metal and eventual failure (Fig. 4). 

Fretting corrosion (36,37) can lead to high contact resistance of base metal contacts, such as tin plate in electronic connectors. Small cycHcal displacements of the connector halves occur because of external vibration or differential thermal expansion and contraction of the mating contacts. The wear debris that is formed remains in the contact zone. The accumulation of oxide debris in the contact region leads to increased contact resistance. Solutions to this problem are stmctures that do not permit movement of contact surfaces with respect to one another, the use of gold as a contact finish, and the appHcation of thick coatings of contact lubricants and greases, which reduce the rate of wear and restrict access of air to the contact surfaces. 

Identification. The primary identif3dng feature is confinement of metal loss to the weld bead (Fig. 15.3), although in advanced stages base metal immediately adjacent to the weld bead may also be affected. Note that this feature seems to distinguish galvanic corrosion of welds from other weld-related corrosion, such as weld decay, which preferentially attacks the immediately adjacent base metal (Fig. 15.4). 

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