Herbert E. Townsend

Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)2), iron trihydroxide (Fe(OH)3), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from Mössbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the Mössbauer subspectral area and recoil-free fraction of each phase. The different Mössbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 µm.

Atmospheric corrosion of different steels in marine, rural and industrial environments

The atmospheric corrosion of the different steels at the different exposure conditions has been investigated by Mossbauer and Raman spectroscopies and XRD. Goethite and lepidocrocite were identified in the corrosion products formed on all the coupons. Magnetic maghemite, which resulted in the high corrosion rate, formed on the carbon steel exposed at the marine site. The inner layer, a protective layer, mainly consisted of interdispersed goethite, and the outer layer mainly composed of interdispersed lepidocrocite. The larger fraction of superparamagnetic goethite, which resulted in decreasing the mean particle size of goethite, in the corrosion products was closely related to reduction in the corrosion rate in the marine and rural sites. The larger amounts of silicon and smaller amounts of phosphorus in the steel increased the fraction of superparamagnetic goethite. However, different amounts of nickel did not affect the formation of the iron oxides after sixteen years of exposure.

Potential-pH diagrams at elevatedtemperature for the system Fe-H2O

Abstract Potential-pH diagrams at temperatures of 60, 100, 150 and 200°C are presented for the Fe-H 2 O system. These have been constructed in the absence of some high-temperature thermodynamic data using the extrapolation method of Criss and Cobble combined with empirical relations for the 25°C entropy of ions. The salient change in the diagrams with increasing temperature is an expanded region of corrosion in high pH media resulting from the increasing stability of the ion HFeO 2 . This change is discussed in relation to (1) cathodic protection, which may become more difficult at higher temperatures and pH, and (2) caustic cracking of steel, which seems to occur where HFeO 2 − is stable.

Intergranular zinc embrittlement and its inhibition by phosphorus in 55 pct Al-Zn-coated sheet steel

Room-temperature tensile and bend tests and Auger electron spectroscopy (AES) were used to study embrittlement in sheet steels coated with a 55 pct Al-Zn alloy and then heated in the range 316 to 538 °C for up to 5000 hours. The results of these studies show that embrittlement is caused by diffusion of Zn from the coating into the ferrite grain boundaries of the steel substrate, reducing intergranular cohesion. The activation energy for grain boundary diffusion of Zn in iron is estimated at 89 kJ/mole. When present in the steel in concentrations of at least 0.04 pct by weight, P is shown to prevent embrittlement by preemptively segregating to the ferrite grain boundaries where it blocks intergranular diffusion of Zn.

Atmospheric Corrosion Behavior of Aluminum-Zinc Alloy-Coated Steel

The influence of the aluminum content of hot-dip aluminum-zinc alloy coatings on their corrosion behavior was studied by means of salt-spray and atmospheric corrosion tests. The objective was to develop an improved aluminum-zinc alloy coating on steel that would be more durable than galvanized coatings and that would be more protective to cut edges and areas of mechanical damage than hot-dip aluminum coatings. The optimum alloy was found to be 55 weight percent aluminum-zinc. This new alloy coating is two to four times as corrosion-resistant as a galvanized coating of similar thickness. Furthermore, for the galvanic protection of cut edges of sheet in some environments, this coating proved to be superior to aluminum coatings.

Zinc-iron phases formed on galvannealed steel

Mössbauer spectroscopy has been used to identify the zinc-iron intermetallic phases present in the coating of three galvannealed steels, two of which were produced on commercial galvanizing lines and one in the laboratory. Both CEMS and XMS have been used in order to determine the depth dependence of each phase. Three main zinc-iron phases have been identified with the proportion of each dependent on the preparation conditions. In the commercially produced coatings, CEMS, probing near the top of the coating, indicates that the zinc rich χ-FeZn13 and δ-FeZn10 alloys are present. XMS indicates the presence of some Π-Fe3Zn10 closer to the steel. In contrast, the laboratory produced sample contains nearly pure χ-FeZn13 through the entire coating thickness. Subphases of the delta and gamma alloys were also identified. These subphases appear to be mixed with a preference for the higher zinc subphase of each to form closer to the surface of the coating.

RESISTANCE OF HIGH STRENGTH STRUCTURAL STEEL TO ENVIRONMENTAL STRESS CORROSION CRACKING

HIGH STRENGTH STRUCTURAL STEEL, ASTM A 514/517, TYPE J, WAS EVALUATED FOR SUSCEPTIBILITY TO ENVIRONMENTAL STRESS CORROSION CRACKING. PRECRACKED CANTILEVER BEAM SPECIMENS, PREPARED FROM 1-IN. PLATE AND LOADED UP TO 90 PERCENT OF THE AIR DETERMINED CRITICAL STRESS INTENSITY, WITHSTOOD 1000 H EXPOSURE TO 3.5 W/O NACL WITHOUT FAILURE. AT LOADS GREATER THAN 90 PERCENT OF THE LOAD REQUIRED TO CAUSE FAILURE IN AIR, FRACTURE OCCURS WITHOUT EVIDENCE OF STRESS CORROSION CRACKING. ADDITIONAL SPECIMENS PREPARED FROM BUTT-WELDED PLATE AND SUBJECTED TO SUSTAINED LOADS BEYOND YIELDING IN 4 POINT BENDING WERE ALTERNATELY IMMERSED IN 3.5 W/O NACL SOLUTION AND DRIED IN AIR FOR 10000 H WITHOUT CRACKING. THESE RESULTS INDICATE VIRTUAL IMMUNITY TO ENVIRONMENTAL STRESS CORROSION CRACKING FOR BOTH BASE PLATE AND WELDMENTS OF THIS QUENCHED AND TEMPERED STEEL.

Transmission Mössbauer Analysis of Nanophased Oxides Formed on High Strength Steels

Nanophased oxides found in the corrosion coatings of atmospherically weathered steels have properties that are scientifically significant and industrially important. Mössbauer spectroscopy proves to be a very useful tool to accurately characterize the corrosion coatings. Samples of carbon steel were exposed in Campeche, along the Gulf of Mexico for up to one year and the development of corrosion products as a function of steel type and exposure time were studied using Mössbauer spectroscopy, micro-Raman spectrometry and X-ray diffraction. Both X-ray diffraction and transmission Mössbauer spectroscopic results indicated that lepidocrocite, maghemite and goethite were the dominant oxides. Transmission Mössbauer analysis at 77 K indicated that for up to three months of exposure, lepidocrocite and maghemite accounted for nearly 80% of the relative amount, with goethite contributing only 20% to the mixture. However, as the exposure time increased to 6 months, the relative contribution of goethite increased at the expense of decreasing amounts of maghemite. Monitoring the environment during the exposure time indicated that the average time of wetness decreased. The decrease in the relative contribution of maghemite to the total oxide concentration is related to the decreasing time of wetness, with increasing exposure time.