Scale on steel

Conductive films such as magnetite (Fe,0 ) or mill scale on steel, and conductive nonmetals such as carbon can function as cathodes when in contact with anodes... 

Other analytical problems to which the direct comparison method has been applied include the determination of mixed iron oxides in the oxide scale on steel [14.10], the beta phase in titanium alloys [14.11], and mixed uranium and plutonium carbides [14.12]. 

The potential of zinc relative to steel is apparently related to the formation of porous Zn(OH)2 or basic zinc salts, which are insulators, under those conditions for which zinc is anodic to steel, but is related to formation of ZnO instead under conditions where the reverse polarity occurs [24]. The latter compound conducts electronically, being a semiconductor. It can, therefore, perform in aerated waters as an oxygen electrode, the potential of which, like mill scale on steel, is noble to both zinc and iron. Accordingly, in deaerated hot or cold waters in which an oxygen electrode does not function, because oxygen is absent, zinc is always anodic to iron, but this is not necessarily true in aerated waters. Apparently, the presence of HCO3 and NO3, aided by elevated temperatures, stimulates formation of ZnO, whereas 01 and SOi" favor formation of hydrated reaction products instead. 

The influence of hydrodynamic conditions on the corrosion rate is not always straightforward. The reason is that often the surface of metals exposed to a corrosive environment becomes covered with corrosion products which impede oxygen access to the surface. As they form, the corrosion rate therefore gradually decreases with time. A typical example of this behavior is the formation of carbonate or sulfate scales on steel exposed to natural waters. 

Control measures can start with elimination of the noise source altogether. Perhaps the procedure can be eliminated altogether or, if this is not possible, another process can be substituted for the one in use. For instance, removal of surface scale on steel with a chipping hammer cotrld be replaced by a chemical treatment, provided this does not result in another substantial risk. 

Fig. IV.6. Adhesion of particles as a function of centrifuging time. 1,2) Adhesion numbers for scale on steel in aviation oil and purified mineral oil (Fdet = 2.2 lo" dyn 3) minimum adhesive force of graphite in MK aviation oil on bronze.

Specify complete removal of mill scale on steel - partial removal is a waste of money. 

Scale (cleaning) A thick layer of oxide that forms on some metals during high temperature processing. Example Mill-scale on steel directly from the steel mill. 

R. L. Higginson and G. D. West, The study of texture development of high temperature oxide scales on steel substrates using electron backscatter diffraction . Mater. Sci. Forum, 495-497, 399 04 (2005). 

S. Birosca, D. Dingley and R. L. Higginson, Microstructural and microtextural characterization of oxide scale on steel using electron backscatter diffraction , J. Microscopy, 213, 235-240 (2004). 

R. L. Higginson, B. Roebuck and E. J. Pahniere, Texture development in oxide scales on steel substrates , Scripta Mater. 47, 337-342 (2002). 

The corrosion rate of steel in carbonic acid is faster than in hydrochloric acid Correlations are available to predict the rate of steel corrosion for different partial pressures of CO2 and different temperatures. At high temperatures the iron carbonate forms a film of protective scale on the steel s surface, but this is easily washed away at lower temperatures (again a corrosion nomogram is available to predict the impact of the scale on the corrosion rate at various CO2 partial pressures and temperatures). 

Metal Treatment. After rolling, the oxide scale on sheet steel is removed by acid treatment (pickling) (see Metal surface treatments). Phosphoric acid, a good pickling agent, leaves the steel coated with a thin film of iron phosphates. This process improves mst resistance but presents a problem if the steel is to be electroplated. 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Heat scales and mst on steel are not of even composition or thickness, and parts are subject to uneven pickling. To minimize over-etching, inhibitors are sometimes used to slow the attack on the clean steel. Many inhibitors produce strong adverse effects if carried into the plating solution. A second electrocleaning step is used after the pickle when inhibitors are employed. 

Oxide scales Oxide scale on alloyed steel 4.218... 

Figure 8.19 F.llingham diagram for the free energy of formation of metallic oxides. (After F. D. Richardson and J. H. F. Jeffes, J. Iron Steel Inst. 160, 261 (1948).) The oxygen dissociation pressure of a given M - MO system at a given temperature is obtained by joining on the lop left hand to the appropriate point on the M-MO frec-energy line, and extrapolating to the scale on the right hand ordinate for POi (atm).

Fig. 7.3 Simplified scheme for the diffusion-controlled growth of multilayered scales on pure iron and mild steel above 570° C...

It has been noted that the total current required to protect large structures can be substantial even in mildly corrosive environments. In seawater, for example, an initial current in the region of 200mA/m for bare steel might well be required in the North Sea. This is because the relatively high oxygen concentration and the tide and wave action all contribute to a facile cathodic reaction. Fortunately this current diminishes with time. The reason for this is the protective scale on the steel surface which forms during cathodic protection by decomposition of the seawater. 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

Thi.s is the most important factor deitermining the life of a protective paint system on steel.. The best surface is one free from mri, scale, grease, dirt... 

Environment Reduce kinetics of cathodic reaction Lower potential of metal Cathodic inhibition Reduce a , reduce O2 concentration or concentration of oxidising species lower temperature, velocity agitation Cathodically protect by sacrificial anodes or impressed current sacrificially protect by coatings, e.g. Zn, Al or Cd on steel Formation of calcareous scales in waters due to increase in pH additions of poisons (As, Bi, Sb) and organic inhibitors to acids... 

Unfortunately, relatively little information has been made available for industrial gas-liquid contactors. Further data from industry could permit significant tests of the reliability of the present correlations and their applicability to scale-up. Steel and Maxon (SI 1) reported on the power requirements during novobiacin fermentation in 20- and 250-liter pilot-plant vessels and in 12,000- and 24,000-gal vessels. The comparative data are difficult to evaluate because of changes that occurred in viscosity and gas retention during the course of the fermentation. In addition, geometric similarity did not prevail... 

Laser cutting, welding, and hot working leave a discolored oxidized layer or scale on the surface of the worked steel. This must be removed in order to perform many of the surface finishing processes. The acid pickling process is used to remove the oxide or scale of metals and corrosion products, in which acids or acid mixtures are used. 

During the hot forming or heat treating of steel, oxygen from the air reacts with the iron to form iron oxides or scale on the surface of the steel. This scale must be removed before the iron is subsequently shaped or coated. One method of removing this scale is pickling with hydrochloric... 

Fig. 4.3.19. Indents on steel surface. Loads 2943, 1962, 981, 490, 245 and 147 mN, scale graduation = 25 fxm, magnification 540 x. (After Leitz-Wetzlar)...

Often, however, steel is subject to pitting attack by sea water. The deepest attack on steel is reported to be about 10 to 15 mils per year. The presence of mill scale on the steel—especially when it covers a large portion of the exposed area—significantly increases the rate of pitting, and penetrations of about 20 mils per year can be expected. This is a result of the mill scale serving as large cathodes to the small, bare anodic areas. Pitting attack often tapers off with continued exposure. 

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