Corrosion related iron oxides : Cu , Mn and Cr doped hematite

We have obtained Cu, Mn, and Cr doped hematites, by a thermal treatment of doped goethites at 500 and 880 °C in air for 24 h. The goethites were prepared in the laboratory from iron chloride solutions in the presence of Cu^" ,̂ Mn^^, and Cr^^ ions. The samples are characterized by Mossbauer spectroscopy and X-ray diffraction. It is found that at 500 °C all the goethites have transformed into hematites. However, the thermal treatment at 880 °C produces hematites with better crystallinity.


INTRODUCTION
It is well documented in the literature that weathering steels produce a protective rust layer due to the presence of the alloying elements Cu, Mn, and Cr.The mechanisms for the corrosion resistance are not well known and studies that shed light into this problem are useful.One way to characterize rust products is Mossbauer spectroscopy where hyperfine parameters are needed to obtain good fits.Hyperfine parameters are modified by alloying elements present in the iron oxides.The knowledge of changes in hyperfine parameters is important to analyze corrosion products from weathering steels.
The transformation by heating under dry conditions, from goethite into hematite has been studied by several authors^ .These studies reveal that the transition temperature usually depends on the crystallinity and the metal su bstitution of the starting material.
It is the purpose of the present investigation to prepare and characterize hematites that have been obtained from the thermal treatment of goethites at two different temperatures.The goethites (a-FeOOH) were synthetized from iron chloride solutions in the presence of Cu "^, Mn "^, and Cr ions.This investigation also includes the influence of Cu, Mn, and Cr on the hyperfine parameters of the hematites.

EXPERIMENTAL
The goethites were synthetized according to the procedure described in references'^ \ Table I summarizes the synthesis procedure to each sample.
The hematites were obtained by heating the goethites at 500 and 880 °C for 24 h in air.The The Mossbauer Spectra were obtained by means of a conventional transmission spectrometer, with constant acceleration, with a ^^Co/Rh source of 25 mCi of initial activity.For the analysis the Mossbauer spectra two computer programs were used: MOSF for the hematite samples and DISTS3E[^1 for the goethites.

RESULTS AND DISCUSSION
Figure 1 shows the room temperature Mossbauer Spectra of the pure goethite and the products of its thermal treatment at 500 and 880 °C.The last spectrum in the figure is that of a commercial hematite, which has been used as a reference sample.It is seen fitting the spectra with a sextet of Lorenzians lines reproduce the experimental data quite reasonably.The derived hyperfine parameters are listed in table II.It is possible to deduce that already at 500 °C the goethite transforms completely into hematite, but the heating process at 880 °C produces hematite with better crystallinity.This latter statement is based on the fact that the line broadening is lower and the hyperfine field is higher in the spectra on the samples heated at 880 °C as compared to the ones treated at 500 °C.It is also worth mentioning that the spectrum of the sample heated at 880 °C is quite similar to that of the commercial hematite.
Figure 2 shows the Mossbauer Spectra of the Mn and Cr goethite before and after the thermal treatment at 880 °C.The spectra of the products with 1 % of each dopant, were reasonably adjusted  with only one sextet.The derived parameters are summarized in table II.It is again observed that the spectra of all the products as well as their derived hyperfine parameters are quite similar to those of the commercial hematite.This means that at 880 °C all the doped goethites have transformed into hematites with good crystallinity.It is important to mention that all the derived hyperfine parameters for the hematites at 1 % of dopant are within the experimental errors the same.The Mossbauer  Spectra of the samples with 5 % Cu and 5 % Mn were acceptably adjusted with one sextet and one doublet; this doublet was reasonably adjusted with a quadrupole splitting of 0.55 mm/s, isomer shift of 0.035 mm/s, and a peak widht of 0.5 mm/s.These parameters were allways kept fxed.However, the origin of this doublet could not be established.In contrast the spectra of sample synthetized in the presence of Cr ions with 5 % and heated at 880 °C seems to become more complex.
A good fit was obtained with 3 sextets.However more measurements are needed, in order to fully identify each one of these components.
Figure 3 shows the X-ray difractogram of the hematite obtained from the sample GcFlOO after heating at 880 °C.This difractogram confirms the presence of hematite in good agreement with the Mossbauer results.

CONCLUSIONS
The procedure to obtain pure and substituted hematite with Cu, Mn and Cr by means of thermal treatment of goethites at temperatures of 500 and 880 °C for 24 h. is reproducible.The Mossbauer parameters have confirmed that in the transformation of goethite into hematite at  880±12 °C for 24 h, the final product is a well crystallized hematite having a line width and hyperfine parameters similar to that of the commercial hematite.The presence of Cr ions perhaps induces a distribution of particles in the hematite, due possibly to the structural substitution of octahedral Fe^^^ for Cr^"^ in the structure.The X-ray diffraction confirms that all samples thermally treated a 880 °C become hematite.

Figure 1 .
Figure 1.Mossbauer spectra of goethite GcFlOO and its thermal treatment products.

Table 1 .
Solutions used for the goethite synthesis Cu (Ka) cathode and filter.The scans were done in the range of 5 ° to 60 ° (29) at a speed of 2° per minute.