Prevention Of Rail Corrosion By Zinc Metallisation

Every year Indian Railway is changing around 4500 km of track. Around 30% of track renewal is on account of Severe Corrosion i.e., about 1300 Tkm/year. Average cost of TRR (P) is around one crore per km and it costs Railway annually 1300 crore approximately. Zinc metallization cost is approx 5 lacs/Tkm. Hence, effective anticorrosive measures are necessary to prevent premature renewals on this account.

            Para 250(b) of IRPWM stipulates Zinc metallization in Severe Corrosion prone areas as per RDSO circular no CT/ACP dated 24.02.06 and in other identified corrosion prone areas, treatment is to be done with bituminous black paint once in a year on inside of gauge face and once in three years on non-gauge face side of rail.

            Zinc metallization has to be done only once in service life of rails in a centralized plant like FBW plant (and not in field) on Web & foot of rails. Free rails need to convert into long rail panels by FBW, duly tested by USFD, before shifting to metallising yard.

            Zinc metallization treatment consists of:-

1.    Surface preparation

v  Cleaning: Removal of grease, paint and other foreign matter by solvents.

v  Sand Blasting: Ballasting with clean & dry abrasive material like sand, crushed steel grit etc.

Ø  Sharp Coarse sand shall be of grain size 600 microns to 1.70 mm; a minimum of 40% to be retained on 850 micron IS Sieve.

Ø  Blasting material should be free from oil & other contamination.

2.    Spraying of Zinc on Rails

Ø  Zinc coating shall confirm to specifications IS: 5905, recommended practice as per IS: 6586

Ø  Uniform Zinc spray with 99.99%. Pure Zinc with Zinc ingots confirming to IS: 209.

Ø  Zinc wire (3.15mm) is fed in high temperature zone of oxy-hydrogen flame just outside the nozzle of the spraying gum throwing compressed air.

Ø  The nominal thickness of zinc coating should be 140-150 microns.

Ø  The minimum local thickness shall not be less than 115 micron (i.e., 75% of nominal thickness).

Ø  The specified thickness should be applied in multiple layers, but not less than two.

Ø  The first layer should be applied within 4 hours of blasting and surface should be finished to specified thickness within 8 hours of blasting.

Ø  The surface after spraying shall be free of uncoated parts or lumps of loosely spattered metal.

Ø  Thickness of coating is to be measured by Elcometer in accordance to IS: 3203-1982.

3.    Etch primer

Ø  RDSO specification recommends etch primer to IS: 5666, Zinc chrome primer to IS: 104 and four coats aluminium paint to IS: 2339, after Zinc Spray.

Ø  As per Para 1.1.1 of IS: 104, Zinc chrome primer is suitable for protection of aluminium and light alloys and may be used for steel work.

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4.    Seal coat painting

Ø  It consists of two components i.e., resin and hardener to be mixed in prescribed proportion advised by manufacturer.

Ø  The mixture should be allowed to mature for 20 minutes after mixing at 27±2⁰ C and should be consumed within 4 hours of mixing.

Ø  This should be applied within 4 to 6 hrs of Zinc coating by spraying method. The thickness of coating should be 50 microns.

Ø  A number of leading brands such as Shalimar, Berger, Asian, Nerolac etc are manufacturing as per IS: 13213.

Life of Zinc Coating

There are two schemes of Zinc Coating specified in IS: 5905 depending upon minimum protection period required:-

ü  Grade ‘A’ Scheme (Minimum protection of 20 years). Zinc coating is for 200 microns followed by Sealer coat.

ü  Grade ‘B’ Scheme (Minimum protection of 10 years). Zinc coating is for 150 microns followed by Sealer coat.

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