Metal Loss Rates Research Articles

Mechanical behavior of erosion-corrosion scales on steels as characterized by single-particle impacts

Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s −1. The steel with higher silicon content showed significantly lower metal loss rates under all conditions. The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s −. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.

Hydrodynamics of disturbed flow and erosion–corrosion. Part II — Two‐phase flow study

AbstractErosion–Corrosion in turbulent, two‐phase liquid/particle flow with recirculation, after a sudden pipe expansion is studied by the application of a numerical flow model along with two different erosion models. The flow model, which is based on a two‐phase flow version of a standard two‐equation model of turbulence and a stochastic simulation of particle‐fluid turbulence interactions, is capable of successfully predicting local values of time averaged fluid velocities and turbulent fluctuations, as well as predicting particle dispersion and particle‐wall interaction. The erosion models used are that of Finnie (1960) and a modified version suggested by Bergevin (1984).The agreement of the predicted and measured hydrodynamic parameters, for flow through a sudden expansion, was satisfactory. Predictions of erosion rates using Bergevin's modified model were in good agreement with the stainless steel erosion measurements for a 2% water/sand slurry flow. The erosion–corrosion model was successful in predicting the overall pattern and rates of metal loss for carbon steel.

Erosion and erosion-corrosion behavior of chromized-aluminized stainless steels at room and elevated temperatures

The metal wastage caused by the impact of small solid particles on surfaces that are simultaneously corroded at elevated temperatures is a problem in several types of energy systems. Protective coatings are used on components in chemical process plants and boilers to protect the surfaces of steel alloys from corrosion, erosion and abrasion, or a combination of these. An investigation of the erosion-corrosion behavior of combined chromized-aluminized coatings on stainless steel 304SS and 410SS was carried out at room and elevated temperature conditions. The coated steels had lower metal loss rates than the bare metal. Different microstructures of the same basic coating material had different erosion-corrosion behavior. Erodent particles that were embedded in the eroded surfaces of the coatings could have influenced their erosion-corrosion behavior.

Erosion-corrosion mechanisms and rates in fe-cr steels

The combined erosion-corrosion behavior of a group of chromium-containing steels used in chemical process and energy production equipment was investigated. It was determined that erosion of oxide scale formed on the surface was the dominant surface degradation mechanism on all of the steels at all test conditions. A change in the scale loss mechanism occurred between the 5Cr and 9Cr steels with the higher chromium steels having a much lower metal loss rate than the 2.25Cr and 5Cr steels. The composition of the scales on the ferritic steels was not as important as their morphology in determining metal loss rates. The presence of Cr 2O 3 in the scales of the austenitic steels helped to reduce scale formation and removal, and the resultant metal wastage.

Erosion-corrosion of steels in simulated and actual fluidized bed combustor environments

The combined erosion-corrosion behavior of steels that are used in coal burning energy generation equipment was investigated over a period of several years using both laboratory nozzle tester and fluidized bed combustor in-service exposures at elevated temperatures. It was determined that the steels tested in both environments had the same types of metal wastage mechanisms. At shallow impingement angles and low particle velocities, scales formed on the steel surfaces that were segmented and were removed by a slow cracking and chipping mechanism. At steeper impingement angles and higher particle velocities, consolidated scales occurred that were removed by a higher rate, spalling mechanism. As the chromium content of the steel was increased, the scale became thinner but was still removed by the same mechanisms. Low chromium content steels containing additional silicon formed a highly segmented scale that protected the base metal, resulting in a low metal loss rate compared with those of much higher chromium content steels.

Erosion-corrosion of protective scales on alloy steels

The need to identify low alloy tubing steels that are resistant to combined erosion-corrosion in fluidized bed combustor environments is an important aspect in the development of this type of coal power generation equipment. An investigation was undertaken to determine the behavior of a number of alloys in erosion-corrosion tests at elevated temperatures. It was determined that a low chromium content ferritic steel (5 wt.%) which contained some silicon (1.5 wt.%) had markedly lower metal loss rates than a ferritic stainless steel with 20% chromium. The reason for the difference was primarily the morphology of the scales. The 5% chromium steel developed a segmented, protective iron oxide coating whereas the 20 wt.% chromium steel had a consolidated iron oxide-chromium oxide scale that locally spalled. Thus, it was determined that some low alloy steels develop an in situ, protective oxide coating in a combined erosion-corrosion environment.

Elevated temperature erosion-corrosion of 9Cr-1Mo steel

The combined erosion-corrosion behavior of 9Cr-1Mo steel (where the composition is in approximate weight per cent) was determined at 850°C using a stream of air and Al 2O 3 particles, 130 μm in diameter, from a nozzle at velocities from 10 to 70 m s −1 at two impact angles, α = 30° and α = 90°. The mechanism of surface degradation and the rates of sound metal loss were determined. Corrosion was the dominant mechanism at all the velocities tested. It was determined that at α = 90° there was a change in the scale loss mechanism at about 30 m s −1 from cracking and chipping to periodic spalling. The spalling increased the sound metal loss rates at the higher velocities. At an impact angle α = 30° the erosion mechanism and rates and the resulting scale morphologies were considerably different from those which occurred at α = 90°.

Relative importance of abrasion and corrosion in metal loss in ball milling

Corrosion studies in laboratory ball mills and grinding environment simulations produce corrision rates and total metal removal rates much lower than those recorded in operating production ball mills. The corrosion component of ball metal loss in many mills probably represents less than 10% of the total loss, judging from AMAX experience and recent US Bureau of Mines (USBM) tests. Such small absolute losses are consistent with ball metal loss measured in production ball mill marked ball wear tests to determine effects of corrosion and abrasion resisting properties of alloys in operating mills. These indicate that metal hardness and microstructure determine relative metal loss rates of a given situation. The combination of ore characteristics and grinding process dynamics determine metal loss severity. Alloying for corrosion resistance and modification of the milling corrosion environment yield relatively small metal loss reductions. Alloying and heat treatment to produce high hardness and microstructures of high carbon martensite and, for cast irons M7C3 carbides, produces the lowest metal losses in typical wet grinding applications.

Metal binding to yeast aminopeptidase I.

Binding of Zn(II) and Co(II) to homogeneous dodecameric aminopeptidase I from yeast was studied. Apparent binding constants were estimated from the dependence of enzyme activity on metal levels in solution as established by metal buffer systems. The binding curves were sigmoidal with Hill coefficients of 1.2-1.6, depending on the metal and on pH. Metal concentrations at half-saturation were 28 nM with Zn(II), and 390 nM with Co(II) at pH 7.0, they increased with decreasing pH. Equilibrium binding studies yielded binding stoichiometries of 0.5-0.6 mol metal/mol subunit for both Zn(II) and Co(II). However, after oxidation of Co(II)-substituted enzyme with H2O2 almost stoichiometric amounts of cobalt (greater than 0.9 mol/mol subunit) were found. The oxidized enzyme was inactive and did not exchange cobalt with the solvent. Native aminopeptidase I was also affected by H2O2, however only after prolonged incubation and at a much lower rate. Apoenzyme modified by ethoxyformic anhydride could not be reconstituted by Zn(II) or Co(II). On the other hand, either metal afforded full protection against ethoxyformic anhydride. This finding, and the pH dependence of stability constants suggest that histidine residues are involved in metal binding. Both the reconstitution of active enzyme from apoprotein and metal and its inactivation due to loss of metal are slow processes with half-times in the minute range. The rate of reconstitution did not depend on Zn(II) concentration. The rates of metal loss were not enhanced by chelating agents containing carboxylate functions. In contrast, chelators coordinating via nitrogen atoms and 2,3-dimercaptopropanol accelerated dissociation. A model is proposed that accounts for the substoichiometric metal contents of aminopeptidase I and for the slow time course of reconstitution. It is assumed that reconstitution involves a slow, monomolecular transition, leading to an active enzyme conformation with lower affinity for metal ions.

Tube erosion by ash impaction

Tube failures occurring in the primary superheaters and reheaters and in the economizers of coal-fired boilers are the result of erosion wear caused by impaction of ash particles. A laboratory assembly was constructed to study the erosion wear of mild steel, and the following parameters were considered; the abrasiveness of ash, the velocity of impacting particles, the angle of impaction and the metal temperature. The velocity was shown to be the most important parameter for tube erosion in coal-fired boilers. For a given concentration of particulate ash in the flue gas the rate of metal loss is proportional to the impact velocity to power 3.5. The wear rates should be negligible when the velocity of flue gas is in the range of 15–20 m/sec, whereas with the values of 30–40 m/sec tube failures are to be expected after 10,000-50,000 h in service. Some remedies to reduce the erosion wear are discussed.

Influence of Oxygen on High Temperature Aqueous Corrosion of Iron

Abstract The corrosion of pure iron in distilled water was investigated over a range of temperature (50–315 C) and oxygen content of the water (<0.1 – 540 ppm). A uniform brown-black coating formed in water of low oxygen content at 260 C. The corrosion rate was low after the initial period. Severe pitting of the iron was encountered in water containing intermediate concentrations of oxygen over the entire range of temperature. The average metal loss rates were also much higher than that obtained in a low oxygen environment. A thin protective film formed when adequate oxygen was present. No significant pitting was noted. The total corrosion at 30 days' exposure was lower for samples in a 26OC–540 ppm oxygen test than for those with very little oxygen at the same temperature. The protective film forming ability of iron in a pitting oxygen-water environment was improved by alloying, particularly with chromium and carbon. The improved corrosion performance of some mild and low alloy steels over pure iron at intermediate oxygen concentrations was also noted. The thin films formed on pure iron in adequately oxygenated water were composed of α-Fe2O3 (hematite) as contrasted with the Fe3O4 (magnetite) coatings produced in deoxygenated water. Distinctive growth habits for the various iron oxides were found on the outer surfaces of the protective coatings.

The Structure of Oxide Scales on Chromium Steels

Abstract The structure of the oxide scale formed on commercial chromium steels containing 5 to 26 percent Cr when oxidized in air or oxygen at temperatures from 700 C to 1160 C for times up to 100 hours were determined by X-ray diffraction methods, supplemented in some instances by chemical analysis. Two distinct types of scale were observed: A type scale occurs when the rate of metal loss is less than approximately 10 mg/cm2/day, and B type when the attack rate is in the excessive range. For exposures near the critical conditions an initial A type scale transforms to B type during oxidation. The essential component of A type scale is Cr2O3. This is usually accompanied by aFe2O3 in an amount which increases with the iron content of the alloy (i.e., with rate of attack). At high temperatures or long oxidation times, dilute solid solutions of each of these phases in the other are formed. When the alloy contains a few tenths percent of Mn, the A type scale may also include copious amounts of MnCr2O4, especially for high chrome alloys and for low temperature oxidations in air. B type scale is more complex than A scale and may be considered as two layers. The outer layer is duplex and is similar in all respects to the Fe3O4 and Fe2O3 layers of the scale on pure iron. The major constituent of the inner layer, corresponding to the FeO layer on pure iron, is a solid solution, FeFe(2–x)CrxO4, of the spinel type. Throughout most of the layer x varies slowly, but near the metal it rises rapidly to x = 2 and near the Fe3O4 layer it falls rapidly to x=0. The mean value of x lies in the range of about 0.5 to 1.5. The greater the chromium content of the alloy, the higher the temperature and the longer the time of oxidation, the larger is the value of x. In cases of very high attack the series of spinels is accompanied by a series of wustites which are modified by chromium additions ranging downward to zero at the Fe3O4 layer. For 10 to 20 hour oxidations at 925 C and at 1000 C the chromium fraction of total metal in the inner layer is about 1.8 times its value in the alloy, whereas the excess is only about one percent in the scale as a whole. The experimental data are compared and contrasted with existing information and are qualitatively interpreted by use of a tentatively proposed Fe-Cr-O phase diagram and a special theory of alloy oxidation. 3.2.3