Researchers develop a new alloy with improved corrosion resistance for Indian railway tracks.
TOILET discharge into the open from passenger trains is perhaps unique to the Indian railway system. Apart from the highly undesirable environmental pollution it causes, it leads to severe corrosion of the rails and their fastenings as the toilet chutes are located almost directly above the rails. It is, in fact, a major cause of corrosion of the rails. This problem is not to be dismissed lightly because the economic cost of corrosion of rails (which, of course, includes normal environmental corrosion) in the Indian context is very significant.
Modification of the toilet discharge system by the Indian Railways, as announced by the Railways Minister a couple of years ago, by replacing the present system with green toilets or aircraft-style vacuum toilets, is certainly welcome, but that addresses only a part of the corrosion problem. Atmospheric corrosion of rails, particularly in coastal regions which accounts for a significant fraction of the Indian network, is also a serious issue requiring urgent attention.
A research paper published in the January 10 issue of Current Science details the development of a novel corrosion-resistant steel by a team of researchers from the Indian Institute of Technology Kanpur (IIT-K), the Research Designs & Standards Organisation (RDSO) of the Indian Railways at Lucknow, the Research & Development Centre for Iron & Steel (RDCIS) of Steel Authority of India Limited (SAIL) at Ranchi, and the Bhilai Steel Plant (BSP). The team was led by the late R. Balasubramaniam, a materials science professor at IIT-K who became well known for unravelling the mystery behind the corrosion resistance of the 1,600-year-old iron pillar in Delhi. He found that the presence of phosphorus in the pillar helped in the formation of a passive film on the surface that provided the pillar with exceptional resistance to atmospheric corrosion. The researchers used the same principle of micro-alloying to arrive at an appropriate composition of alloying elements for the steel that could be used to make corrosion-resistant rails.
The development of a new alloy with relatively improved corrosion resistance compared with the standard rail currently in use was undertaken as an academia-industry-user collaborative research project under the Technology Mission on Railway Safety (TMRS) launched in 2003 by Prime Minister Atal Bihari Vajpayee. The Indian Railways, which operates the second largest rail network in the world, runs over 11,000 trains every day, 7,000 of which are passenger trains. The network spans 1,08,706 km and carries about 14 million passengers daily from 6,853 stations across the country. The rail is the most important component of a rail system. Therefore, rail failure due to corrosion over such a large network is naturally an important aspect of passenger safety.
The rails in use are made of steel containing 0.7 per cent carbon (C) and 1 per cent manganese (Mn) and are called C-Mn rail steel. It is a wear-resisting grade steel and is commonly referred to as Grade 880 rail, or 90 UTS rail, corresponding to a tensile strength of 880 mega pascal (MPa), or 90 kg/mm {+2}. According to the Indian Permanent Way specification, the C-Mn rails are expected to have a life of 800 gross million tonne (GMT), which is equivalent to about 12-13 years under normal traffic conditions. Corrosion is estimated to reduce their life to nearly half, and the annual loss as a result of pre-replacement of corroded rails is estimated to be about Rs.440 crore.
Data analyses for 2000-07 show that only 32 per cent of rail replacement was carried out after completion of the normal expected lifetime. Data also show that 37 per cent of the rails are replaced owing to corrosion before the estimated lifetime, whereas only 16 per cent of the replacement is owing to wear and 15 per cent owing to weld failure.
The second component of a rail track is the pre-reinforced concrete sleeper, whose heavy weight provides stability to the entire track structure. Between the rail and the sleeper is a grooved rubber pad (made of natural rubber, styrene butadiene or poly butadiene rubber). The pad provides insulation, absorbs vibrations and impact, and increases the coefficient of friction between the rail and the sleeper to make the contact between the sleeper and the rail stronger. Then there is the rail-fastening system to fasten the rails to the sleepers. This consists of elastic rail clips (ERCs), which are made from silico-manganese spring steel and serve to dampen the vibrations, and spheroidal graphite (SG) cast iron that is cast inside the sleeper. The leg of the ERC is inserted into the SG. The ERC keeps the rail anchored to the sleeper by transferring the load on to the foot of the rail. However, it does not directly touch the rail, but a liner made of mild steel or of non-conducting glass-filled nylon, in case of electrified rails, placed between the rail and the ERC serves as the interface. The liner prevents the rail foot from getting damaged following the impact of the ERC.
Corrosion is degradation of an engineering material due to an electrochemical reaction with the environment. In any electrochemical process, there are four components: anode, cathode, electrolyte and an electrical connection between the anode and the cathode. Corrosion or metal loss or metal oxidation always occurs at the anode, which is the electrode from which the current (or positive charges) flows out into the electrolyte.
Atmospheric corrosionThe most common form of corrosion of rails is atmospheric corrosion caused by the wetting due to moisture in the atmosphere and its drying. The severity of atmospheric corrosion is a function of the moisture residence time and the frequency of wetting and drying. However, the good thing about atmospheric corrosion is that it is generally uniform and so does not pose any safety problem. Pollutants and contaminants in the atmosphere destabilise the protective rust on the surface and dictate the extent of atmospheric corrosion. So, for instance, the presence of chloride ions in coastal areas accentuates atmospheric corrosion. Hence, rails near coastal regions are more prone to environmental corrosion and need more frequent replacement than rails in a dry area. Since one cannot alter the prevailing atmosphere, economic loss due to atmospheric corrosion is an issue that needs to be addressed by the use of corrosion-resistant materials, which would delay replacement of rails as much as possible.
The second kind of corrosion, which is more important for both economic and safety reasons, is enhanced localised corrosion. A dangerous form of localised corrosion is crevice corrosion, which occurs between the liner and the rail foot. In the Indian context, there are two chief causes for localised corrosion. One is the leakage of current in electrified tracks, which is termed as stray current corrosion'. An electric locomotive draws current from the overhead traction lines and the return path for this current is through the rails to the generating substation. Rails are adequately shielded or insulated from the earth so that the least resistance path for the return current is through the rails alone.
If for some reason the current gets discharged from the rail to the earth at some spot, severe localised corrosion will occur at that anodic region, leading to rapid metal loss, thinning and even perforation of that rail section. A technique called bonding is used by the Railways to tackle this problem. One would have noticed a strip of mild steel joining both the rails to an anode, which is usually a waste metal lump, buried inside the earth adjacent to the tracks. If there is a stray leakage from the rail, the current gets discharged through the (dispensable) buried anode and the rail gets protected from stray current corrosion. Of course, significant reduction of the problem can be achieved by proper design of the railway electrification system itself. But in case of the other major source of localised corrosion in the Indian context, namely corrosion owing to toilet discharge, the critical issue is the material composition of the rail.
According to S. Srikanth of the RDCIS, one of the researchers involved in the project, while corrosion due to stray current discharge is widely documented in published literature, there is no literature available from other rail systems of the world on corrosion due to open toilet discharge. Given the length and breadth of India, long-distance travel by trains is quite common, which invariably results in grater quantities of lavatory discharge. The discharge affects only the rail foot facing the inside of the track the gauge side and not on the non-gauge side. Initially, we did not believe this human role in the increased corrosion of Indian rails, says Srikanth.
Toilet discharge on to the rails and collection of moisture from the atmosphere cause intense localised corrosion under the liners. This leads to the thinning and perforation of the rail foot under the liners, resulting in premature failure of the rails, which is a significant safety concern. This localised corrosion below the metal liners is commonly referred to as crevice corrosion. A crevice is any location in the system that has limited access to the atmosphere.
As Balasubramaniam explained in an article he wrote on rail corrosion shortly before his death, this results in the formation of oxygen concentration cells. The region where oxygen is depleted inside the crevice becomes anodic with respect to the rest of the material that is exposed to the atmosphere. This leads to an intense attack at the crevice location because the electrons required by the large cathodic area outside the crevice are supplied from the small anodic area within the crevice. This is the corrosion process whereby metal atoms get converted into metal ions. The process is autocatalytic, and according to the authors of the Current Science paper, the attack is not easily visible to the eye. Crevice corrosion, too, gets accentuated in the presence of chloride ions, which are present in coastal environments and in toilet discharge from trains.
Combating crevice corrosion at the location of the liner is, however, not straightforward. One of the ways of doing this for in-service rails is to prevent the locations below the liners from coming into contact with the environment. The simplest way to do this is to apply a protective coating on the surface. The Indian Railways tried this method initially. Field trials on different coatings were conducted at a corrosion-prone location near Visakhapatnam. Polymeric coatings were found to degrade owing to the atmosphere in that environment. Zinc coating (galvanising) was found to give the best performance.
Zinc coating acts both as a barrier against the environment and as a sacrificial anode (cathodic protection). So zinc coating of rail foot is an effective method to combat crevice corrosion. However, besides practical problems in implementing this in field conditions, it is a costly option.
Developing corrosion-resistant rails based on new alloy chemistry, which will perform better than the rails currently in use, is a viable alternative approach to tackle the problem. This is what Balasubramaniam and associates did in the mission-oriented project that was carried out over a period of about four years. As Balasubramaniam emphasised, corrosion-resistance does not mean no corrosion. If that is required, then rails will have to be produced from extremely expensive materials such as stainless steel. Here, the aim of developing corrosion-resistant materials is only to increase the life of rails significantly and delay their replacement.
The basic principle of micro-alloying rail steel is that the alloying element(s) will induce passivity in iron. It is also clear that small quantities of alloying elements will not alter the basic mechanical properties of the rail steel drastically. Only a few elements, added to steel in minor quantities, are known to improve its corrosion resistance. These are chromium (Cr), nickel (Ni), copper (Cu), silicon (Si), molybdenum (Mo) and phosphorus (P). According to the paper, these elements are effective in improving corrosion resistance where the surface, after periodic wetting due to rain or dew, is dried easily by sunlight. The excellent resistance to atmospheric corrosion due to the phosphorus in the Delhi iron pillar exemplifies this property. The paper also points to an interesting fact that the same alloying elements that are normally added to improve the mechanical properties of rail steels also induce passivity in iron.
SAIL had been conducting experiments in micro-alloying of rail steel with Cu and Mo from the knowledge that Mo offered resistance to localised corrosion. SAIL developed a novel Cu-Mo rail steel in collaboration with the RDCIS and the BSP, and it has been undergoing field trials. The typical composition of Cu-Mo rail is 0.69 C, 0.24 Cu, 0.18 Mo. According to Srikanth, the superior corrosion performance of Cu-Mo rails has been adequately demonstrated both in the laboratory and through field trials at Marripelam near Visakhapatnam in the East Coast Railway (ECoR) division and at several corrosion-prone sections in the South Central Railway (SCR) divisions over a five-year inspection period. The corrosion-resistant nature of the rust coating in Cu-Mo rails has been found to be due to the formation of protective magnetite (one of the several iron oxides; formula Fe {-3}O {-4}) on the rail surface.
However, despite the promise of Cu-Mo rails, the high and fluctuating cost of Mo (nearly all of which has to be imported) is an economic disadvantage of large-scale production of Cu-Mo rails. So in developing the new rail steel, the pricing element was kept in mind, says the paper. In this regard, chromium and copper are cheap and abundant.
The team carried out experiments on different combinations of minor alloying elements with a view to determining the synergistic effect of these elements on the corrosion properties of rail steel. It was important to add the optimum amount of these micro-alloying elements such that they remain in solid solution and provide corrosion resistance. At the same time the effect of these alloying elements on the mechanical properties and processing of rails had also to be considered in the design of compositions, the authors write.
Table 1 gives the compositions of the four new rail steels and the plain C-Mn rail steel. According to the paper, the rail steel plates were processed at the RDCIS using the same rolling parameters as those used at the BSP. The performance of these novel rail compositions, in particular their resistance to localised corrosion, was assessed through a variety of tests.
IRS-T-12 lays down the specifications for the mechanical properties of steel rails for the Indian Railways. It was found that these novel steels met the IRS-T-12 specifications with regard to mechanical properties as well. In fact, both Cu-Mo and NCC 90 UTS rails have already been included in the 2009 revision of IRS-T-12.(NCC steel is an alloy containing Cr, Cu and Ni.) Although the rails have been alloyed for achieving improvements in atmospheric corrosion resistance (which occurs through a process of weathering and protective rust layer or patina development), there are accompanying improvements in strength, hardness, wear, fracture toughness and fatigue resistance under cyclic loads, which have been substantiated by us through specialised laboratory material characterisation techniques, points out Srikanth.
According to the paper, in acidic and near-neutral conditions, the rates of corrosion of all the alloys were similar under complete immersion test, and therefore one could not judge the comparative effectiveness of corrosion resistance among the different alloying compositions. To differentiate between them, samples were subjected to alternate wetting and drying, and the surface layers forming on these alloys were evaluated by the technique known as electrochemical impedance spectroscopy (EIS). This showed that NCC steel performed best.
In addition, says the paper, fretting wear studies also indicated superior resistance of this composition compared with the others. In view of its improved corrosion performance, this TMRS project recommended the rail steel with micro-alloying additions of 0.59 per cent of Cr, 0.40 per cent of Cu and 0.20 per cent of Ni (Table 1) for rail manufacture.
On the basis of the recommendation, 120 tonnes of NCC steel rails were processed at the BSP in June 2007 and 50 tonnes were welded and laid over a half-kilometre stretch in the Vijayawada-Gudur section in September 2008. Since then, NCC rails were laid in several sections in the SCR in a staggered manner until May 2009. Also, 500 tonnes of these rails were processed and laid over corrosion-prone sections in the ECoR.
In April 2009, the Indian Railways ordered 10,000 tonnes of NCC steel for more detailed studies spread over a larger region. According to Satya Prakash of the RDSO, who also is part of the project team, about 80 per cent of this has been rolled and delivered. Some NCC rails were laid four months ago in the Guntakal division. So far, NCC rails have been laid over a total track length of 7.75 km in real field conditions. The corrosion behaviour of these rails will be monitored for five years before a conclusion is reached on their field performance, says Srikanth. Alongside, another 1,000 tonnes of Cu-Mo rails has been ordered, which will be rolled in the next financial year, says Satya Prakash.
Long-term testing of rails in a simulated environment corrosion chamber has confirmed the superior corrosion resistance of NCC rails. More recently, samples from actual NCC, C-Mn and Cu-Mo rails were evaluated specially for their crevice corrosion resistance, according to the paper. Since localised corrosion under the liner results in the loss of material, the roughness of the surface is an indication of the material's corrosion performance the rougher the surface, the higher the degree of attack. The crevice corrosion attack was found to be the least in NCC compared with other rail steels (Table 2). The researchers have also determined that the enhanced protection of NCC rail was owing to the higher amount of magnetite and goethite (termed delta-FeO(OH)) in the rust. The rust formed in NCC was found to be more compact and hence more protective in nature.
In terms of the economics of these novel steel rails, the cost could increase by about 25 per cent, says Satya Prakash. Though it was the cost factor that drove the project towards finding novel compositions in lieu of Cu-Mo rails, the difference in the cost between NCC and Cu-Mo is perhaps not very much. Compared with an increase of Rs.10,000 per tonne for Cu-Mo steel over conventional C-Mn rail steel, the cost per tonne of NCC will be about Rs.8,000 higher.
Nickel too is imported and that is the reason for an almost similar ballpark cost for NCC steel. However, in view of the higher lifetime of these novel rail steels, in terms of life-cycle costs these should prove more economical and cost-saving in the long run for the Indian Railways.
The Indian Railways are convinced of the performance of both Cu-Mo and NCC but the additional cost of alloying with Mo or Ni is definitely a deterrent from an organisational point of view, says Srikanth. They will like to have a fair assessment about the overall service life, achievable GMT and other techno-economic factors before taking a decision on procuring rails in sizable quantities from SAIL.
Also, the dilemma that possibly confronts the Railways as to which will be a more economically viable option: whether to use corrosion-resistant weathering rails or to design/revamp their toilet discharge disposal systems on passenger trains. Since one does not contradict the other, especially in view of atmospheric corrosion, which anyway has to be combated, the Railways would be well advised to push ahead on both fronts.
