Steel Guru : Phosphoric Iron

The Gupta period Delhi Iron Pillar near Kutub Minar in New Delhi, India is testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron. This Pillar has attracted the attention of corrosion technologists because it has withstood corrosion for the last 1600 years.

The presence of relatively high phosphorus in the range of 0.25% in the forge welded Delhi Iron Pillar plays a major role in its excellent corrosion resistance. The presence of phosphorus leads to the formation of a protective passive film on the surface, which provides the Pillar its exceptional corrosion resistance properties. However, in modern steel making process, the phosphorus content is controlled to 0.05% because phosphorus segregation to grain boundaries reduces ductility of steel.

A detailed study was undertaken by a PhD student, Gadadhar Sahoo, under the guidance of Professor R Balasubramaniam of the Materials and Metallurgical Engineering Department of Institute of Technology of Kanpur India to understand possible industrial applications of phosphoric irons. The first aim was to render the phosphoric irons ductile and the second aim was to locate a modern application wherein the corrosion resistance of phosphoric irons could be put to good use.

Ductility an issue in steel

Ductility is a very important property from the point of practical application of any engineering material. It is known that phosphorus segregation to the grain boundaries makes these locations weak and results in poor ductility.

Phosphorus will not be present in regions where carbon is located in the iron matrix because phosphorus is a substitutional solute element whereas carbon is an interstitial solute element, which is achieved by maintaining a small amount of carbon in phosphoric irons. Intelligent use of the phase transformations in the iron phosphorus system was used to locate these carbon atoms along the grain boundaries to keep the phosphorus atoms away from these locations.

Iron phosphorus phases

The phase diagram of iron-phosphorus shows a (alpha + gamma) dual phase region at high temperature. Phosphoric iron in the composition range 0.25 and 0.50 % P, is soaked in the two phase region (temperature between 1000^oC and 1100^oC), then austenite phase (g) will precipitate on the grain boundaries of ferrite phase (a).

This is well known in physical metallurgy. Austenite has a higher solubility for carbon than phosphorus and therefore all the carbon is pushed to the grain boundary region while the phosphorus is removed from the grain boundary region. After a suitable soaking time at high temperature, the phosphoric irons can be air cooled to room temperature. The beneficial aspect of this treatment is that phosphorus, which was removed from the grain boundary regions, does not go back to these regions during air cooling because phosphorus requires time to diffuse to the grain boundary regions. In this manner, a high temperature soaking in the two phase region and subsequent air cooling should result in good ductility for phosphoric irons.

UTS

Tensile testing of triplicate samples indicated good ducitlities for phosphoric irons, especially P1 and P2. In the same figure, the tensile test result of a commercial reinforcement bar used for concrete reinforcement applications is also shown.

DUCTILITY

Good levels of ductility were obtained for phosphoric irons because phosphorus was kept away from the grain boundaries. This was confirmed by microscopy. In this figure, the optical micrograph of P2,that was soaked in the (alpha + gamma) dual phase region is shown, after etching with Nital.

The light contrast at the grain boundaries indicates locations of low phosphorus content. Interestingly, this structure is known as a “ghost” structure because although the entire structure is ferrite at room temperature, the location of prior austenite and prior ferrite is revealed by the contrast that obtains due to differences in phosphorus contents. Well, this is not important anyway here, but the idea of “ghosts” does also apply in metallurgy!

Application in rebars

Rebars used for reinforcement of concrete in structural applications need to possess the necessary strength, ductility and corrosion resistance and the prevalent technology uses thermo mechanical treatment to impart these properties.

Phosphoric irons can be processed by a similar method utilizing the existing arrangements, with the major difference being the ingot soaking, bar quenching and further cooling arrangements have to be fine tuned to produce phosphoric iron with a tough surface and a strong interior. This can be achieved by suitable design criteria.

Corrosion resistance testing

The corrosion behavior of phosphoric irons has been evaluated in a wide variety of environments. It was concluded that phosphoric irons possess necessary corrosion resistance in near neutral and alkaline conditions. But mainly the corrosion resistance of phosphoric irons was evaluated extensively in simulated concrete environments and compared with commercially available material.

A wide variety of techniques was used to study corrosion in concrete conditions. These included polarization methods linear & Tafel polarization, potentiodynamic polarization and potentiostatic polarization, electrochemical impedance spectroscopy, salt fog immersion, complete immersion and atmospheric immersion experiments. Apart from samples exposed to aqueous solutions, cement-grouted samples were also tested to simulate actual application conditions.

The results of the detailed study indicate the superior corrosion resistance of phosphoric irons in concrete conditions. The corrosion rate of normal TMT was much higher compared to phosphoric irons in addition phosphoric irons exhibited good passivity in concrete environments. Phosphoric irons also showed excellent corrosion resistance even in presence of chloride ions.

In order to visually show the good corrosion behavior of phosphoric irons, the result of immersion testing in saturated Ca(OH)₂ solution containing 5 % NaCl is shown below.. The nature of surface after 125 days of immersion can be noted for three phosphoric irons and two commercial grades of reinforcement steels from TMT and CRS.

The severe nature of corrosion in case of P1 , TISCON and CRS is quite clear. Phosphoric iron P3 is the most resistant, followed by P2 and then P1. These visual images clearly prove the beneficial effect of phosphorus in conferring corrosion resistance, especially in simulated concrete pore solution containing a relatively high concentration of aggressive chloride ions.

Inference

This significant result indicates that phosphoric irons will provide good service in environments where chloride ions are present, like in case of structures near seacoast. To increase the life of concrete structures, corrosion resistant rebars, having traces of Cu & Cr are used on the coast line, which can be easily replaced with phosphorus iron bars. An added advantage is that they will not be costly compared normal TMT bars.

Conclusion

The idea for developing phosphoric irons originated from the study of the Metallurgical Wonder of India – the Delhi Iron Pillar and now we believe in the old adage that “The best of the new is often the long forgotten past.”

The commercial trials for making phosphorus iron rebars are yet to take place but the patent for the idea and technology has been applied in most of the important countries.

References

R. Balasubramaniam, “On the Corrosion Resistance of the Delhi Iron Pillar,” Corrosion Science -, 42 (2000) 2103-2129.
R. Balasubramaniam and A.V. Ramesh Kumar , “ Characterization of Delhi Iron Pillar Rust by X-ray Diffraction, Fourier Infrared Spectroscopy and Mössbauer Spectroscopy,” Corrosion Science , 42 (2000) 2085-2101.
R. Balasubramaniam, “ Phosphoric Irons for Concrete Reinforcement Applications,” Current Science , 85 (2003) 9.
Gadadhar Sahoo, “Corrosion of Novel Phosphoric Irons for Concrete Reinforcement Applications,” PhD. Thesis, IIT Kanpur, 2006, submitted.
R. Balasubramaniam and Gadadhar Sahoo, Corrosion Resistant Phosphoric Iron for Concrete Embedment and Reinforcements Application ,