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Corrosion Behavior

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Corrosion Resistance of Zn-Al Alloys

Zinc-aluminum alloys exhibit exceptional corrosion resistance across diverse environments, with aluminum content significantly enhancing zinc's inherent protective properties. This article examines the corrosion behavior of various Zn-Al alloys (including ZA-8, ZA-12, ZA-27, and No.3) in salt spray, pH-varying solutions, atmospheric conditions, water environments, and chemical exposures.

Duplex Stainless Steels: Part One

Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength.
Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; hence they have a "duplex" structure. This effect is achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel.

Duplex Stainless Steels: Part Two

Duplex stainless steels (DSS) are widely used in chemical, petrochemical, cellulose and nuclear plants around the world. Because of their fine austenitic-ferritic microstructure they offer an excellent combination of mechanical and corrosion resistance properties.

Galvanic Corrosion

Galvanic processes occur between different metals and between different areas of the same metal in the water environment. Water is an electrolyte, a poorly conductive one at the low dissolved solids content of fresh waters, and a highly conductive one at the high dissolved solids content of sea water.
When two different metals are immersed in an electrolyte and connected through a metallic path, current will flow. Oxidation occurs at the anode and reduction (normally oxygen reduction) occurs at the cathode.

Heat-Resisting Alloys

Heat-resisting alloys useful at temperatures above 1200oF are based on iron, on nickel and on cobalt and contain elements that form precipitates that harden the matrix after solution treating and aging. Structural stability and resistance to oxidation and corrosion at elevated temperatures are required of these alloys.

Heat-resisting steels

Steels are now used for a wide variety of conditions entailing heat and corrosion under both static and dynamic stresses, such as aero engine valves, furnace conveyers, retorts, oil cracking units and gas turbines. Three important properties are necessary in material used at elevated temperatures:
1. Resistance to oxidation and to scaling.
2. Retention of strength at the working temperature.
3. Structural stability as regards carbide precipitation, spheroidisation, sigma formation and temper embrittlement.