Corrosion carbon steel section

Investigators determined that a carbon steel section of 6-inch (150 mm) line was installed in an area in which specifications required corrosion-resistant 5-percent chromium—1/2-percent molybdenum (generally called 5-chrome) alloy piping. In a process plant like a fluid coker, the materials used are a mixture of carbon steel and other steel alloys. Apparently, the welder and the maintenance crew, who previously repaired this piping some time ago, were not aware of the piping specifications. Apparently, they did not realize that if a material like carbon steel was installed in an area requiring 5-chrome alloy piping, erosion or corrosion could cause failure. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Carbon dioxide produces a solution of carbonic acid (as in boiler condensate, see Section 53.3.2). Carbon steel is often employed but corrosion rates of up to 1 mm/yr can be encountered. Coatings and non-metallic materials may be employed up to their temperature limits (Section 53.5.6). Basic austenitic stainless steels (type 534) are suitable up to their scaling temperatures. 

The discussion that follows will be concerned mainly with materials in the first of these categories. Moreover, since the corrosion of cast iron is discussed elsewhere (see Section 3.6), and since little wrought iron is produced nowadays, the subject matter will virtually resolve itself into the corrosion of ordinary carbon steels, as used in mass for general purposes. The corrosion of low-alloy steels and that of stainless steels are considered in Sections 3.2 and 3.3 respectively. 

Stress-corrosion cracking (Section 8.10) New metal/environment combinations which produce stress-corrosion cracking are continually being found. Combinations discovered in service in recent years include titanium in red fuming nitric acid carbon steel in liquid anhydrous ammonia and in... 

Pitting (Sections 1.5 and 1.6) Pitting of carbon steel is seldom catastrophically rapid in service and can often be accommodated within the corrosion allowance for the equipment. It often takes place under scale or deposits so that regular descaling of equipment can be beneficial. 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

Partial what-if analyses for the two example processes described in Section 4.0 are shown in Tables 4.9 and 4.10. Although for actual, more complex analyses, the what-if tables for each line or vessel would be separate, for these examples, a single table was developed. A preliminary hazard analysis (PHA) would identify that the intrinsic hazards associated with HF are its reactivity (including reactivity with water, by solution), corrosivity (including carbon steel, if wet), toxicity via inhalation and skin contact, and environmental toxicity. The N2 supply system pressure is not considered in this example. The specific effects of loss of containment could be explicitly stated in the "loss of HF containment" scenarios identified. Similarly, the effects of loss of chlorine containment, including the reactivity and toxicity of chlorine, could be specified for the second example. 

The basic material of construction in both sulfuric and hydrofluoric acid units is carbon steel. Normally, neither acid is corrosive to carbon steel at temperatures below 150° F., which covers the reactor section in both types of units. 

The reaction and acid storage sections of an HF alkylation unit are constructed of carbon steel, stress-relieved, and X-rayed. When properly constructed, these sections are essentially free of corrosion. Monel trim is used on all valves, pumps, and instruments. 

Partial blocking effect was first identified for pure iron in contact with aerated sulphuric acid medium [55]. Corrosion of carbon steel in sodium chloride media clearly showed the porous layer effect (see Section 5.2) [74]. The same effect was found for zinc corrosion in sodium sulphate [75] and the properties of the layer which was demonstrated to be formed of an oxide/hydroxide mixture were further used for building a general kinetic model of anodic dissolution [76], usable for measurement of the corrosion rate from impedance data. 

Flow sheets of the chemical plant were prepared with a pre-design of the main equipment. For the capital cost assessment, the factored estimate has been chosen because it considers characteristics of the process corrosive products, high temperature (up to 850°C for Section II) and high pressure (up to 50 bar for the helium coolant). The factored method is essentially based upon charts and formulas developed over 30 years in the petroleum and chemical industries in France (Chauvel, 2000). It consists of estimating costs of basic equipment (generally carbon steel) and correcting them for materials factors. 

The petroleum feedstocks that contain sulfur as an impurity are handled under ambient conditions in carbon-steel tanks and pipelines where the corrosion attack by sulfur is less severe. In the desulfurization step, vaporized feedstock is processed at 400°C in the presence of hydrogen sulfide (H2S) and carbonyl sulfide (COS) - both of which are highly corrosive. Stainless Steels (SS) 304, 316 or 321 (for the fired heater) are used as the material of construction for various pieces of process equipment in this section of the plant. Equipment failures occur because of external corrosion and thinning of fired-heater coils and interior deposition of carbon from coking which leads to overheating. Fuel-gas lines, that contain hydrocarbon vapors and H2S, should be constructed of SS 304 and heat traced to avoid condensation88. 

The corrosive deterioration of metal surfaces incurs a great cost to the worldwide economy. Accordingly, there have been many research efforts devoted to understanding the surface chemistry behind these reactions. As we have already seen, this has led to the development of a number of useful alloys that are sufficiently resistant to corrosion - through spontaneous formation of protective oxide layers. However, for other less resistant metals such as carbon steels, a protective layer must be postdeposited onto a metal surface in an effort to prevent corrosion. In this section. 

Uniform corrosion usually occurs in fairly aggressive environments that attack the whole surface. Examples include carbon steel in seawater or acids, or aluminum alloys in strong alkali. The rate of metal loss is usually rather high, but, because it is distributed over the whole surface, the performance can usually be predicted, and managed with corrosion allowances, in most situations. Thus, sheet steel piling is often used in seawater without any corrosion protection, the corrosion rate of around 0.1 mm/yr, coupled with the relatively thick steel sections, giving an acceptable life. 

A reaction section, where operations are conducted in a reactor fed in descending stream with a mixture of p-xyiene, acetic acid and catalyst solution prepared in a separate device. The reaction medium is agitated by the introduction of air at the bottom. The corrosive action of bromine and organic acids on carbon steels makes it necessary to use special, stainless materials (Hastelloy C), both for the reactor and for certain parts of the equipment, particularly the heat recovery system. The temperature and oxygen content of the reaction medium must be carefully controlled to prevent the formation of undesirable side products. The heat of reaction is removed by vaporization of part of the reaction medium (acetic acid, p-xylene and water), and by condensation and reflux to the reactor. Residence time is about one hour, and the yield is up to 95 molar per cent... 

---