Unalloyed Steel

In unalloyed steel containers formamide discolors slowly during shipment and storage. Both copper and brass are also subject to corrosion, particularly in the presence of water. Lead is less readily attacked. Aluminum and stainless steel are resistant to attack by formamide and should be used for shipping and storage containers where the color of the product is important or when metallic impurities must be minimized. Formamide attacks natural mbber but not neoprene. As a result of the solvent action of formamide, most protective paints and finishes are unsatisfactory when in contact with formamide. Therefore, formamide is best shipped in containers made of stainless steel or in dmms made of, or coated with, polyethylene. Formamide supphed by BASF is packed in Lupolen dmms (230 kg) or Lupolen canisters (60 kg) both in continental Europe and overseas. 

It is agreed generally that the characteristics of the rust films that form on steels determine their resistance to atmospheric corrosion. The rust films that form on low-aUoy steels are more protec tive than those that form on unalloyed steel. 

The distinguishing feature of the behaviour of the slow-rusting low-alloy steels is the formation of this protective rust layer. Corrosion in conditions where it cannot form is little different from that of unalloyed steel, although the particular alloying elements present will have some influence on the actual rate at which corrosion occurs. 

Unalloyed steel can be directly oxidized by steam at temperatures over 750 °F (399 °C). Extensive cracks may occur, the steel surface may resemble tree bark, and magnetic oxide deposits are generated at the failure site. Intergranular oxidation may take place. In the thermal oxidation reaction, hydrogen is directly released as shown ... 

This is rather unique and is completely different from stress-corrosion cracking of unalloyed steel in alkaline solutions. In alkaline solutions the cracks always run between the crystals and usually only after plastic deformation. Normalized, stress-relieved steel is not usually subject to cracking254. 

The nail samples are similar to hair in many respects but the total amount available is usually small. The samples are invariably collected using Ni-coated unalloyed steel clippers or scissors. The specimens should be scraped further using blades made of quartz or forceps coated with teflon. Cleaning of the samples to remove fat, sweat and dirt can be carried out by soaking in suitable solvents, e.g. acetone, ethyl alcohol (Mahler et al., 1970) or in a non-ionic detergent (Harrison and Tyree, 1971). The samples should then be rinsed with a copious flow of deionised water and finally dried in a clean atmosphere at a temperature of 60-90°C. 

Several specimen were tested for each of the coating types listed in Table 7.3. In this sub-section, however, the term SPECIMEN No. X is also used for COATING TYPE No, X and should be so interpreted according to the context. The material used for the coated steel specimens were unalloyed steel St 37, Crevice and pitting corrosion testst and galvanic corrosion tests with copper as a contact matcrialj were performed on all specimens supplied,... 

Specimen 7 displays irregular corrosion spread over the entire surface of the specimen. It takes the form of wide depressions with pitting corrosion. The test solution is rust-brown and cloudy with a rust-coloured sediment, the main component of which is ferric hydroxide. This picture is typical of contact corrosion between unalloyed steel and copper in sodium chloride solutions (Figure 11.85(g)). 

In the case of unalloyed steel, the corrosion rate drops after a long test period and, with no change in medium, and reaches a constant value. The corrosion rate for copper increases with time if the medium is not changed. 

Unalloyed steels contain essentially only carbon as alloying element, and the carbon content has a major influence on the properties (Table 2.10-3). The carbon is present in the form of iron carbide FcjC which, together with the iron matrix, forms the structural material steel. 

Shearing a rod section of hot-rolled round steel (unalloyed steel with low carbon content) with a mass tolerance of 1.5%... 

A negative influence on corrosion resistance in sulfuric acid is observed with increasing carbon content. The corrosion rate of iron in sulfuric add is too high for the successful application of unalloyed steels [15]. 

Fig. 13 Influence of oxygen concentration on the corrosion rate of unalloyed steel in salt solution at 25 °C (free convection) (a) 1.5 x 10 M CaCh (b) solution with KCl ( ), Na2S04 (o), NaCi (x) [17].

Material-enviromnent combinations giving stable passivity are dealt with more extensively in later chapters (7, 8 and 10). Important examples are unalloyed steels in aerated alkaline aqueous solutions without significant contents of aggressive ions like Cr, stainless steels and titanium in oxidizing environments over a very wide range of pH (but stainless steels are also sensitive to chlorides) and aluminium in natural atmospheres and waters. 

Sueh surfaee layers ean reduee the real eorrosion rates eonsiderably. One example will be mentioned here a elean iron or unalloyed steel surfaee in stagnant air-saturated water would eorrode at a rate of 0.6 mm/y at 20°C, and in flowing water up to several times more, while the average eorrosion rate in free seawater really is only 0.1-0.15 mm/y due to deposition of rust and salts, partieularly CaCOs. 

Schmitt G, Rothmann B. Studies on the corrosion mechanism of unalloyed steel etc.. Part I. Kinetics of the liberation of hydrogen. Advanees in CO2 Corrosion, Vol. 1, NACE, Houston, 1984 72. 

Parallel use of the Pourbaix diagram and overvoltage curves is sometimes useful when evaluating how typical and stable the uniform corrosion is under different conditions. Let us consider a case with iron or unalloyed steel in two different environments ... 

Example 1 Copper accelerates eorrosion eonsiderably on unalloyed steel and even more on aluminium alloys in seawater beeause Cu is a more efficient eathode than steel and partieularly aluminium. 

As already pointed out, the area ratio between flie metals in a galvanie eouple plays a crucial role for the galvanie eorrosion rate. The importanee is best illustrated in a potential-log current diagram, where flie area of the less noble metal eonveniently can be set equal to 1 cm. In Figure 7.8 this is shown by an example where iron or unalloyed steel in one case is alone = 1 cm ) and in other eases is eonneeted to stainless steel so that the total eafliodic area is 10 and 100 em, respeetively. For the sake of simplieity it has been assumed that flie cathodic overvoltage eurves are equal on stainless and unalloyed steel. Two different environments are eonsidered ... 

In this connection one should be aware of the possibility to protect critical parts of a system (equipment, plant) by means of less noble and less critical structural components in the same system, i.e. the corrosion is localized to preferred parts of the system. For instance, stainless steel pipes, fittings, pumps and valves in seawater systems can be protected against serious deterioration by use of pipe sections of unalloyed steel or cast iron. These sections must be easy to replace, or they must have relatively large wall thickness and/or large exposed surface area in relation to the more noble materials. In heat exchangers, relatively thick tube plates of steel can be used in combination with tubes of a copper alloy. 

Rapid corrosion because of deposits has also occurred in other environments, e.g. on boiling tubes for in-evaporation of glue water in a fish-glue production ship. The temperature varied from 40°C to 100°C from stage to stage in the plant. Heavy corrosion attacks were found both on unalloyed and on stainless steel AISI 304. At least partly, the attacks were caused by insufficient cleaning. An example of such an attack on unalloyed steel is shown in Figure 7.25. 

The outlet from a tank made of stainless steel includes a short unalloyed steel tube that is insulated from the tank and formed as shown in Figure 1. There is a water flux of 600 litres/min through the outlet. The corrosion rate dependence on flow rate is schematically shown in Figure 2. It is found by experiments that ... 

Determine the corrosion current density and the corrosion rate in mm/year in the narrow and in the wide part of the unalloyed steel tube under each of the following conditions a)-d). Illustrate each case with schematic anodic and cafliodic potential-log current curves, and indicate on the curves the potentials in die respective parts of the tube. 

A copper pipe and a pipe of unalloyed steel are joined as shown in Figure 3. Water with a resistivity of the order of 300 ohm cm is flowing through the pipeline. Assume that the oxygen reduction reaction is diffusion controlled on both materials, and that the dissolution of steel is activation controlled. Compare with a couple of examples in Seetion 7.3 (Figures 7.13 and 7.11). 

A tank for storing an aqueous solution has a bottom made of stainless steel and a mantel made of unalloyed steel. The bottom and the mantel are welded together. The mantel and the weld are painted. Explain what happens if a small defect (a holiday ) arises in the paint coating on the unalloyed steel. How could this have been avoided (There are various ways.)... 

Figure 5 Joint between a pipe of unalloyed steel and a pipe of stainless steel carrying seawater.

Where abrasive impact wear occurs in hammer mills, such as those used for comminution of raw coal in power stations, continuous replacement of plain plates of unalloyed steel is preferred to the application of substantially more expensive materials with only moderately improved service lives. 

A practical example of almost uniform surface corrosion is as follows A pipe made of unalloyed steel St 35 used at approximately 90°C for the transport of service water showed material erosion of the inner surface after 3 years in operation. A layer of corrosion product (mainly iron oxide) had formed on the inside with a practically constant thickness over the entire area. Given the operating conditions, the material corrosion had to be due to oxygen corrosion. 

Influence of Grain Interface Cementite on Stress Corrosion Behavior of Mild Unalloyed Steel ... 

FIGURE 20.36 Current density-voltage curves of unalloyed steels in alkaline solution and position of stress corrosion cracking. 

FIGURE 20.37 Stress corrosion cracking in an unalloyed steel storage tank for liquid ammonia. 

Unalloyed steels can be used in air up to 550°C and low-alloy steels up to approximately 600°C. The applicability of high-alloy steels is determined by the alloy contents, with special importance attached to Cr, Si, and Al, as demonstrated in Figures 20.48,20.55, and 20.56. Water vapor and carbon dioxide in air generally worsen the scaling behavior of steels. The resistance of steels in water vapor is of particular importance in steam boilers and heat exchangers. It has been investigated in the literature at temperatures up to 800°C. 

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