Susceptibility steels

With these definitions in mind, a systematic classification has been made. The various types of embrittlement found in refineries and petrochemical plant equipment, susceptible steels, basic causes, and common remedies are listed in the accompanying table. 

Types of Embrittlement Temperature Range Manifestations TVpical Service, Equipment Structures Involved Susceptible Steels Basic Cause(s) Common Remedies... 

Strain aging can also occur in susceptible steels by welding near a crack, causing embrittlement (called dynamic strain aging). For this and other reasons, many users require that plate material intended for pressure-containing components be scanned by ultrasonic inspection equipment for cracks and flaws near edges to be welded. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

Corrosion Inhibitors. Steel-reinforcing wire and rods embedded in concrete containing quinoline or quinoline chromate are less susceptible to corrosion (72) (see Corrosion and corrosion control). Treating the surface of metals with 8-hydroxyquinoline [148-24-3] makes them resistant to tarnishing and corrosion (73). Ethylene glycol-type antifreeze may contain quinoline, 2-chloro-, 4-amino-, 8-nitro-, or 8-hydroxyquinoline to prevent corrosion (74). 

Manufacture. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother Hquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide (341,342). In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkaH. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Siace dimer acids, monomer acids, and trimer acids are unsaturated, they are susceptible to oxidative and thermal attack, and under certain conditions they are slightly corrosive to metals. Special precautions are necessary, therefore, to prevent product color development and equipment deterioration. Type 304 stainless steel is recommended for storage tanks for dimer acids. Eor heating coils and for agitators 316 stainless steel is preferred (heating coils with about 4s m (50 ft ) of heat transfer surface ia the form of a 5.1 cm schedule-10 U-bend scroU are recommended for a 37. 9-m (10,000-gal) tank. Dimer acid storage tanks should have an iaert gas blanket. 

Cathodic treatment of steel parts in acids could be expected to contribute significantly to hydrogen embrittlement of the part if the steel has been previously heat treated to over 40 HRC. Cold-worked steel is susceptible at a lower hardness. Some work shows more embrittlement from the plating bath than from preplate treatments (36). 

In addition, the magnetic characteristics of a material can change as a function of stress (e.g., unannealed series 316 stainless steel can be magnetic after machining), temperature, pressure, and physical and chemical treatment. Therefore, when two paramagnetic materials with similar magnetic susceptibilities are to be separated, the possibility that pretreatment will facilitate subsequent separation should be studied. 

Area effects in galvanic corrosion are very important. An unfavorable area ratio is a large cathode and a small anode. Corrosion of the anode may be 100 to 1,000 times greater than if the two areas were the same. This is the reason why stainless steels are susceptible to rapid pitting in some environments. Steel rivets in a copper plate will corrode much more severely than a steel plate with copper rivets. 

Metals that produce protective oxide layers (such as stainless steels) are especially susceptible to crevice attack. The reduced oxygen concentration in the crevice inhibits repair of the protective oxide film. This is especially true if acidic anions are present, which further retards oxide repair. Stainless steels containing molybdenum are usually less susceptible to attack. 

Zinc is susceptible to attack from oiQ gen concentration cells. Shielded areas or areas depleted in oxygen concentration tend to corrode, forming voluminous, white, friable corrosion products. Once the zinc layer is breached, the underlying steel becomes susceptible to attack and is severely wasted locally (Figs. 5.12 and 5.13). 

Alloys whose corrosion resistance depends on forming a protective oxide layer, such as stainless steel, are susceptible to severe localized attack when pH falls as a result of nonoxidizing acid excursions. How-... 

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

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