Aqueous steel corrosion

Specifications, Shipping, and Analysis. Hydrogen fluoride is shipped in bulk in tank cars (specification 112S400W) and tank tmcks (specification MC312). A small volume of overseas business is shipped in ISO tanks. Bulk shipments are made of anhydrous HF as well as 70% aqueous solutions. A small amount of aqueous solution may be shipped as 50%. Cars and tmcks used for anhydrous HF transport are of carbon steel constmction. It is possible to ship 70% aqueous in steel from a corrosion standpoint however, mbber lining is commonly used to eliminate iron pickup, which is detrimental to product quaUty in a number of appHcations. Hydrogen fluoride of less than 60% strength must always be shipped in lined containers. 

In principle, cathodic protection can be applied to all the so-called engineering metals. In practice, it is most commonly used to protect ferrous materials and predominantly carbon steel. It is possible to apply cathodic protection in most aqueous corrosive environments, although its use is largely restricted to natural near-neutral environments (soils, sands and waters, each with air access). Thus, although the general principles outlined here apply to virtually all metals in aqueous environments, it is appropriate that the emphasis, and the illustrations, relate to steel in aerated natural environments. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Iron and Stainless Steel. The purpose of XPS investigations on typical corrosion systems like iron or stainless steel, is the determination of the composition of the passive surface layer, if possible, as a function of depth. As a consequence of the technical and economic relevance of corrosion reactions, XPS investigations on corrosion systems are numerous. With respect to the application of XPS, there is no difference between corrosion systems and any other electrochemical surface reaction like oxide formation on noble metals. Therefore, in this paragraph only a few recent typical results of such studies, using XPS, will be mentioned. For a detailed collection of XPS corrosion studies the reader is referred to references [43,104], A review of aqueous corrosion studies, using XPS, was given by McIntyre for the elements O, Cr, Mn, Fe, Co, Ni, Cu and Mo [105], The book edited by M. Froment [111] gives an impression of the research achieved on passivity of metals up to 1983. 

Aqueous corrosion is electrochemical in nature and involves the passage of electric currents. The current enters the solution at local anodes and leaves the solution at cathodic areas on the metal. In the case of steel, ferrous ions enter the solution at the anode and hydrogen is deposited at the cathode. 

Type 317 has the highest aqueous corrosion resistance of all AISI stainless steels. 

Chemical process plants often use epoxy, acrylic, polyurethane, and other coatings to prevent corrosion.107 Food cans often have epoxy resin linings. A coating made by the plasma polymerization of a 1 1 perfluorobutane-hy-drogen mixture protected copper from aqueous corrosion.108 Auto makers often use electrogalvanized steel... 

Anodic undercutting does not significantly contribute to the loss of paint adhesion from steel under normal aqueous corrosion conditions even when painted steel has been subjected to substantial anodic currents, little or no adhesion loss has been observed (19-21). Anodic undercutting has been reported to be of... 

Figure 47. Effect on aqueous corrosion of 8001 aluminum at 260°C, 7 m/sec velocity of added surface of aluminium or stainless steel (82)...

The extent of aqueous corrosion often depends on the presence of impurities and trace contaminants in the water present. For example, carbon-steel reinforcing bars in concrete corrode more severely in acidic conditions and in the presence of chloride ions, a process called electrochemical attack. On the... 

The most common nonelectrochemical approach for the study of corrosion is weight loss measurements. Such measurements are limited by the resolution of the gravimetric device, and, for aqueous corrosion, are usually applied in aggressive environments such as the boiling acids used to evaluate sensitization of stainless steels in ASTM A262 [76]. The Quartz Crystal Microbalance (QCM) is a gravimetric instrument capable of submonolayer sensitivity that has been increasingly applied over recent years in the area of corrosion [85, 86]. 

An alkyd resin containing less than 1% PANI was examined for its ability to protect carbon steel against aqueous corrosion. In field tests, in urban and marine environments, as well as in accelerated laboratory tests, the presence of PANI in the alkyd resin improved the corrosion protection of carbon steel and also the degradation resistance of the coating [214]. 

Aromaa J, Ronkainen H, Mahiout A, Hannula SP (1999) Identification of factors affecting the aqueous corrosion properties of (Ti, Al) N-coated steel. Surf Coating Technol 49 353-358... 

The special case of the bimetallic effect between a zinc coating and the substrate that it is protecting is discussed under hot water aqueous corrosion resistance as the normal bimetallic effect whereby zinc protects steel is reversed in some waters, usually at 60-90°C. Bimetallic corrosion of zinc occurs mainly when zinc or zinc-coated steel is protecting uncoated steel or other base metals such as copper. Many of the uses of zinc deliberately invoke this principle, but in other cases an unwanted effect arises as a result of constructional requirements, and avoidance of bimetallic corrosion is needed. 

In aqueous corrosion, raising the temperature increases the dissolution of zinc in water. A marked increase occurs up to around 60°C followed, by a decrease at higher temperatures due to the decrease in oxygen supply and the formation of more compact and adherent scale. Intergranular corrosion of zinc casting alloys is a risk above 70 C in wet or humid conditions, such as in steam, when no protective layer can form and selective dissolution of the structure occurs. In hot hard waters, scale forms at water temperatures above 55 C. This scale has a coarse grain structure and does not adhere well to the zinc surfece. Corrosion of the zinc occurs locally because of the discontinuities in the scale or local electrochemical action. Above 60°C, zinc usually becomes cathodic to steel therefore, the steel will corrode to protect the zinc coating. 

The behavior of weathering low-alloy steels in aqueous corrosion tests and applications is unpredictable. In 1953, early tests on weathering steels containing copper, chromium, phosphorus, and nickel showed superior... 

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