D. C. Cook

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.

The role of goethite in the formation of the protective corrosion layer on steels

The corrosion products formed on carbon and weathering steels exposed in marine, industrial and rural environments in the United States for 16 years have been investigated using Mössbauer spectroscopy, Raman spectrometry and chemical analysis. Mössbauer spectroscopy was used to measure the fraction of each oxide in the corrosion coatings and micro-Raman spectrometry was used to locate and map the oxides to 2 µm spatial resolution. Mössbauer spectroscopy identified the corrosion products in the weathering steels as 75% goethite, 20% lepidocrocite and 5% maghemite. Raman analysis showed that the corrosion products generally formed as alternating layers containing different oxides. For the weathering steels the protective inner-layer closest to the steel substrate consisted of nano-sized goethite ranging in size from 5–30 nm and having a mean particle size of about 12 nm. The outer-layer close to the coating surface, consisted of lepidocrocite and goethite with the former oxide being most abundant. Electron probe micro-analysis measured significant chromium in the goethite close to the steel substrate. Comparison of the goethite in the corrosion products was made with synthetic chromium substituted goethite with nearly identical microstructural characteristics being recorded. It is concluded that chromium inclusions in the goethite are important for formation of a nano-phase oxide layer which may help protect the weathering steel from further corrosion.

Strain induced martensite formation in stainless steel

The Conversion Electron and X-ray Mössbauer studies of the surface of Type 316 stainless steel at 400 K, 300 K, and 100 K show that both the substitutional and interstitial elements perturb the cubic symmetry at the iron site. The single peak of austenite is a superposition of at least five quadrupole split doublets whose magnitudes and intensities depend on the type and concentration of the impurity elements. However, when the surface of the stainless steel is plastically deformed, a layer of martensite about 5000 Å thick is formed on the austenite base. This layer consists of a mixture of 31 pct martensite with the rest being the original austenite. The magnetic environment of the iron in this martensite is controlled by the concentration of alloying elements, and the distribution of the hyperfine fields is determined by the number of nearest and next nearest neighbor impurity atoms. The magnetic field decreases linearly at first as the number of nearest neighbors increases and then follows a nonlinear trend for a number of nearest neighbors. The temperature dependence of the sublattice magnetization is different for each number of neighbors, and a Curie temperature has been estimated for each site.

Influence of attrition milling on nano-grain boundaries

Abstract Nanostructured materials have a relatively large proportion of their atoms associated with the grain boundary and the method used to develop the nano-grains has a strong influence on the resulting grain boundary structure. In this study; attrition milling iron powders and blends of iron powders produced micron-size particles composed of nano-size grains. Mechanical cold-working powder resulted in dislocation generation; multiplication; and congealing that produced grain refinement. As the grain size approached nano-dimensions; dislocations were no longer sustained within the grain and once generated rapidly diffuse to the grain boundary. Dislocations on the grain boundary strained the local lattice structure which; as the grain size decreased; became the entire grain. Mechanical alloying of substitutional aluminium atoms into iron powder resulted in the aluminium atoms substituting for iron atoms in the grain boundary cells and providing a grain boundary structure similar to that of the iron powder processed in argon. Attrition milling iron powder in nitrogen gas resulted in nitrogen atoms being adsorbed onto the particle surface. Continued mechanical milling infused the nitrogen atoms into interstitial lattice sites on the grain boundary which also contributed to expanding and straining the local lattice.

Mössbauer effect and XRD studies of iron-zinc binary alloys

Mössbauer spectroscopy and XRD were employed to characterize the microstructural properties of iron-zinc binary alloys between 0–31 at.% Fe. Samples were prepared with accuracies of ±0.5 at.% Fe, and the Mössbauer and lattice parameters were monitored as a function of iron concentration across each phase. Two iron sites were observed in the Γ phase (18–31 at.% Fe), whose occupancies and isomer shifts varied continuously with iron content. However, the quadrupole splitting of each site remained constant. Within the Γ1 phase (19–24 at.% Fe), three iron sites were observed whose isomer shifts and quadrupole splittings remained constant, while their occupancies varied with iron concentration. For the first time, a third iron site was observed in the δ phase (8–13 at.% Fe), whose occupancy increases with iron content. Also, the site occupancies of the two other δ sites appear to remain constant, while other Mössbauer parameters vary continuously with iron content. Analysis of the ζ phase (6–7 at.% Fe) showed the presence of one iron site, whose parameters were not observed to change due to the small variance in iron concentration. XRD studies indicate the lattice parameters across the Γ and δ phases vary continuously with iron concentration. Moreover, a better understanding of these phases, as formed in galvanneal steel coatings, was obtained.

bct-Fe formation during mechanical processing of bcc-Fe powder

In this study, a novel technique for producing nanocrystalline bct-Fe from bcc-Fe is reported. bct-Fe, often referred to as martensite, is normally produced either by a thermal transformation or through a shear stress mechanism from retained fcc-Fe. The authors produced nanocrystalline bct-iron-carbon/nitrogen phase by processing bcc-Fe powder in high-energy ball mill. bct-Fe formed after a significant amount of mechanical processing (cold working) in the presence of interstitial atoms of either carbon or nitrogen, bct-(Fe-C/N). The authors hesitate to call the bct-Fe phase observed in this study martensite because martensite is normally though to form from fcc-Fe and from planar lattice shifts resulting in a relationship in the martensite lattice orientation and the bcc/fcc lattice in which it is embedded. No such relationship between the bct formed in this study and the bcc matrix was observed. The capability of producing nanocrystalline bct powder offers the possibility of producing near-net-shape, high strength products.

ATMOSPHERIC CORROSION IN THE GULF OF MEXICO

The corrosion products on steels exposed at two sites in Campeche, México and one site at Kure Beach, USA, have been investigated to determine the extent to which different marine conditions and exposure times control the oxide formation. The corroded coupons were analyzed by Mössbauer, Raman and infrared spectroscopy as well as X‐ray diffraction, in order to completely identify the oxides and map their location in the corrosion coating. The coating compositions were determined by Mössbauer spectroscopy using a new parameter, the relative recoilless fraction (F-value) which gives the atomic fraction of iron in each oxide phase from the Mössbauer sub‐spectral areas. For short exposure times, less than three months, an amorphous oxyhydroxide was detected after which a predominance of lepidocrocite (γ-FeOOH), and akaganeite (β-FeOOH) were observed in the corrosion coatings with the fraction of the later phase increasing at sites with higher atmospheric chloride concentrations. The analysis also showed that small clusters of magnetite (Fe3O4), and maghemite (γ(Fe2O3), were seen in the micro-Raman spectra but were not always identified by Mössbauer spectroscopy. For longer exposure times, goethite (α-FeOOH), was also identified but little or no β-FeOOH was observed. It was determined by the Raman analysis that the corrosion products generally consisted of inner and outer layers. The protective layer, which acted as a barrier to slow further corrosion, consisted of the α-FeOOH and nano-sized γ-Fe2O3 phases and corresponded to the inner layer close to the steel substrate. The outer layer was formed from high γ-FeOOH and low α-FeOOH concentrations.

Conversion electron and X-ray Mössbauer studies of the corrosion products and surface modifications in stainless and weathering steels

The study of the corrosion products formed on the surface of Type 316 stainless steel exposed to chlorinated seawater for 18 days shows that ferrihydrite and lepidocrocite are the first oxides to form. The surface also shows the formation of a martensite when it is cleaned by light brushing before exposure.Samples of weathering steel, ASTM A242 Type 1, exposed for 51/2 years to either a marine or inland rural environment have been studied between 100K and 400K using CEMS and XMS. Different amounts of hydroxoxides are present on the surface of each sample. The steel exposed to the inland environment contains more α-FeOOH.

MOSSBAUER EFFECT DETERMINATION OF RELATIVE RECOILLESS FRACTIONS FOR IRON OXIDES

The relative recoilless fraction (F-value) of each of six iron oxides, defined as the ratio of the recoil-free fractions of two different materials, was experimentally determined relative to hematite at 300 K and 77 K by Mossbauer spectroscopy. Using the relative recoil-free fractions compared to that of hematite, the relative recoilless fractions between all pairs of the seven iron oxides were determined. The F-values can allow conversion of Mossbauer subspectral areas to the relative atomic, molecular, or weight fractions of each iron oxide present in a mixed oxide phase sample.