IT was a dull October morning in the leafy London surburbs. The road was quiet apart from a few early commuters stepping out into the damp air. Hardly the description of an extreme environment.
However, a few metres away lay a harsh and unforgiving environment, unassuming and unnoticed by most who walked or drove over it every day. The combination of level crossing and the busy West Barnes Lane resulted in such a corrosive environment that a replacement was required after only three months.
The rail sits between rubber pads as it runs across the level crossing. However, traffic works grit, road salt and water into the gap between the rubber and the rail, which is held against and abrades the rail with each passing vehicle. Add stray current in the running rails into this cocktail and the result is near perfect conditions for rapid rail foot corrosion.
This was an ideal location to test Tata Steel's corrosion protection coating, Railcote. On October 11 2009 the rails were replaced with rails protected with Railcote on the rail foot and web.
The rails were inspected in July 2011 and were found to be in good condition. At the last inspection in August 2014, 58 months after installation, they were still in good condition.
Railcote has protected the rail in the level crossing in two ways; firstly as a barrier preventing the water and oxygen necessary for corrosion reaching the steel of the rail and secondly by corroding in preference to the steel, an electrochemical reaction, known as sacrificial protection. This means that if there is any damage to the coating, the Railcote around the point of damage corrodes rather than the exposed steel.
This process is applicable in other environments where temperatures and conditions are quite different from London. In the salt pans, or sabkhas, of the Arabian peninsula, daytime temperatures can range from 30-55oC with high humidity, while at night temperatures drop significantly. Add into the mix a salt crust with mud underneath as well as periodic flooding in some locations and it is another textbook environment for corrosion.
One of the challenges of building a railway across the salt pans is to slow down rapid corrosion of the rails caused by the combination of water, salt and heat. As a result it makes sense to use a coating to protect the rail foot.
The story of Railcote's development began over 10 years ago when Tata Steel (then Corus) launched an epoxy coating. Since then experience of in-track performance has informed research and development of test coatings that improve protection in various track conditions.
One challenge is getting any coated rail transported to site and installed with minimal damage to the coating. For example, epoxy coatings give very good barrier protection but are often brittle and susceptible to damage during installation which in certain conditions can accelerate corrosion in the damaged area and this can develop into a pit which acts as a stress raiser causing rail cracking. As a result an aluminium metal spray coating was developed with very good resistance to mechanical damage. However, this metal spray coating does not protect the rails effectively when there is stray current in the track. So the search was on for a coating to provide protection all conditions.
The testing regime included accelerated corrosion testing (prohesion), simulated stray current corrosion assessments, natural exposures and impact testing. The prohesion test is a standard test that takes place in a cabinet that goes through alternate hours of wet and dry cycles for 2000 hours.
The wet cycle uses a weak salt solution that is atomised to create a fog followed by a dry cycle at 35oC. The test samples were fabricated from 100mm by 150mm by 5mm thick hot rolled mild steel panels, which were grit blasted to Sa2.5 with a 67-75 µm profile and then coated. To simulate the damage often found on coated rails after they are installed, each sample had a 2mm wide by 40mm long defect gouged through the coating down to the metal substrate.
The prohesion test results demonstrated that all coatings provide some protection over uncoated steel. The epoxy coating samples sustained heavy corrosion and deep pits in the damaged areas. This result suggested that while the coating would perform well if installed and maintained without any mechanical damage, severe corrosion would result where any damage did occur.
The aluminium metal spray samples showed a high level of creep back again indicating a low tolerance to any mechanical damage in service. The Railcote coated test panels were virtually free of under film corrosion after the 2000th test confirming that the coating provides effective anodic protection and prevents corrosion of the damaged area.
The coating's impact resistance is determined by dropping a calibrated steel weight attached to a 15mm steel ballbearing onto the surface of the coating. The height at which the weight is dropped determines the impact energy in joules using the formula E = mgh where:
E = the impact energy in joules
m = mass of the impactor in kg
g = acceleration due to gravity (9.806650), and
h = drop height of indentor.
The failure of the coating is determined when the impactor penetrates the surface of the coating through to the metal substrate.
Table 1 shows that at approximately 4J the 5mm-thick steel panels start to sustain plastic deformation which further added to the severity of the test. The Railcote and aluminium metal spray coatings deformed with the substrate and did not flake off. However, the epoxy coating cracked at about 2J.
A test was devised to simulate the stray current conditions found on track. A 50mm plastic tube was glued to the coated steel sample which became the anode. Builder's sand was put into the tube and a mesh cathode inserted to within 15mm of the coated steel sample. A 1% sodium chloride solution was poured in to act as the electrolyte and allow a circuit to form between the mesh cathode and the steel sample anode. A potential of 500mV was set up between the mesh cathode and the steel anode using a dc power unit and the test was run for 28 days. On the epoxy coating sample the corrosion undercut the coating, the aluminium metal spray sample lost all the aluminium coating exposing the steel substrate which rusted. The Railcote showed up to 20% coating loss and some blisters.
These results show that the epoxy coatings give good protection provided the coating is intact and undamaged. However, any damage to the coating leads to accelerated corrosion in the damaged area and cracking occurs when subject to impact over 2J. The aluminium coating has a high impact resistance. However the results indicate a relatively large creep back distance from any damaged area. Under stray current conditions there is relatively rapid loss of coating surrounding the damaged area, which would then lead to accelerated corrosion of all the exposed area. The Railcote performed well in all laboratory tests providing good corrosion protection of the damaged area without blistering or creep back. The coating also performed well in stray current environments with only a 20% consumption of the coating surrounding the damaged area.
Experience from using the coatings in track offers valuable lessons which define the product specification and are directing future research and development. It is difficult to avoid some damage to coatings during installation and maintenance, so a coating that has good resistance to damage and still provides corrosion protection when damaged is ideal. Networks often have areas of stray current, and coatings need to be able to protect here too. This is why our research pointed to Railcote, a coating with good impact resistance that provides sacrificial protection even when it is damaged.