GALFAN. an improvement for galvanizing process protections. Product Technical Department

Transcription

1 an improvement for galvanizing process protections Product Technical Department

2 INTRODUCTION Steel, the most used of all metals, is cheap, strong and versatile, but it has a tendency for wearing away as time goes by if not adequately protected. Corrosion is an electrochemical phenomenon which takes place when n there is a different electrical potential between two points connected by an electrolyte (e.g. water with an acid content). Such difference in potential for a steel wire in natural environments (e.g. a river), may be brought about by a difference d in salt or oxygen concentration around the wire surface itself. The main effective method to overcome this problem consists in galvanizing g the wire by a hot dipping process which ensures a zinc layer firmly alloyed with steel and a proportionally longer wire life.

3 INTRODUCTION The zinc coating protects the steel for two reasons: 1) ISOLATING it from the external corrosive agents 2) through a SACRIFICIAL CORROSION (in place of the main metal) ) in case of contact. In addition to this, the galvanized wire, in case of breaching or scratching of the coating, is subject to a much slower corrosion compared to the uncoated steel l wire, since the zinc is a poorer and chemically less effective anode. Infact even if the breaching in the zinc coating puts the metal in contact with the atmosphere, the zinc acts anyway as the anode and therefore is subject s to corrosion (but it still corrodes slower when compared to steel), while the steel is the cathode. So corrosion of zinc produces hydroxides and carbonates which cover the exposed steel surface blocking any corrosion phenomenon (white rust)

4

5 WHAT IS GAL - MAC GAL Galvanisation Fantastique Steel wire corrosion protection with a zinc (~95%) aluminium (~5%) alloy Small additions of Cerium-Lanthanum improve the reliability of the coating process Maccaferri Mission: The development and the improvement of innovative solutions and products dealing with erosion control problems

6 Why does the coating improve the steel corrosion resistance? Due to the alloy structure Due to the aluminium passive barrier protection Because the high performances of zinc and aluminium (as passive barrier) can be combined

7 THE STRUCTURE is an eutectic zn-al mishmetal: : a combination resulting in a more uniform crystalline microstructure and in improved mechanical characteristics. The scanning microscope analysis shows the zinc coating laminar microstructure and fine microstructure

8 THE BETTER ALLUMINIUM PASSIVE BARRIER 1. Aluminium is more electropositive than zinc, with lower corrosion rates than zinc 3.The aluminium oxides crystals are more uniform and less porous therefore the passivating performance is more effective 2. A passive barrier is a product s ossidation that can protect the steel 4. The aluminium oxides run off resistance, improves their percentage compared to the zinc one, so exalting the passivating phenomena

9 WHY THE ZINC-ALUMINIUM ALLOY? Zinc has a cathodic feature which promotes its sacrificial protection: when zinc is scratched, damaged or abraded away, zinc oxides cover the exposed steel surface The steel-zinc alloy is therefore important for the internal protection stability The aluminium oxides are externally more stable. IS ABLE TO ADD THE SINGLE POSITIVE FEATURES OFFERING A BETTER CORROSION RESISTANCE

10 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS The improved resistance to corrosion and the consequent increase in durability have been proved qualitatively by INDEPENDENT tests in a wide range of corrosive environments. Corrosion test procedures: Salt Spray test ASTM B 117 SO 2 Kesternich test ASTM G1/88

11 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS Centre de Recherches Métallurgiques - Abbaye du Val-Benoit - Liege -Belgium -(1987)

12 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS Centro Sviluppo materiali - Unità di Ricerca - Roma (1997)

13 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS Salt Spray test conditions ASTM B DIN SS Conditions: - 100% Relative Humidity - 35 C - 5% NaCl (Sodium Chloride) Control every 24 hours End of the test at 5% red rust ASTM G1/88 Kesternich - SO 2 Sulphordioxide Test Conditions Conditions: 1 cycle: -88 hours - 40 C -100% -1l SO 2 addition -16 hours -Room temperature (18-28 C) -NaSO 2 addition -Max.75% rel 100% relative humidity rel.. humidity End of the test at 5% Red Rust

14 AGEING TESTS Ageing tests have demonstrated that the percentage of aluminium on the external surface of products increases with time. This development at the corrosion front, the decreasing of zinc and the increasing of alumnium,, in connection with an increasing of oxygen at the same locations, gets stronger with longer corrosion endurance. LINE-SCAN: Fe - Al - Zn LINE-SCAN: Fe - Al - Zn Reference: Bergische Universitat Gesamthoschschule Wupperalt

15 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS Reference: NISSHIN STEEL CO. Ltd.

16 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS performance ratio (GPR) as compared to heavy galvanizing PERFORMANCE RATIO (GPR) AS COMPARED TO HEAVY GALVANIZING The GPR increase with exposure time as seen in this diaghram.. It shows twice as effective as heavy galvanized at 8-1/28 years, 2-1/22 times better at 15 years, 3 times at 25 years and almost 4 times better at 50 years. GPR YEARS 60

17 EXPOSURE TESTS IN DIFFERENT ENVIRONMENTS performance ratio (GPR) (GPR)) = Zn loss of Heavy Galvanized coating Zn loss of coating

18 DURABILITY CONCLUSIONS The exposure test results have substantiated our assumptions that s corrosion resistance progressively improves over time We can say that the protection is 4-5 times durable than the zinc one Therefore it can be enounced that gabions and Reno mattresses in hexagonal wire mesh have an indefinite life expectancy because the durability of the wire is longer than consolidation/integration between the structure and the surrounding soil

19 THE EXISTING SPECIFICATIONS ASTM 856 Zinc/5% Aluminium Mishmetal alloy coated carbon steel establish standard specifications for zinc/aluminium alloys protection The new EN Eurocode (draft) for steel wire products considers both heavy zinc and coatings

20 CONCLUSIONS AVAILABILITY ILZRO DOCUMENT NEW BROCHURE MARKETING ILZRO = International League Zinc Research Organization

21 PRODUCTS

22 PRODUCTS

23 EXPOSURE TO CORROSIVE ENVIRONMENT The improved resistance to corrosion and the consequent increase in durability have been proved qualitatively by indipendent tests in a wide range of corrosive environments. Weight loss (gr/m 2 ) Coating weight (gr/m 2 ) Hours of exposure in 10 ppm SO 2 atmosphere (weeks) Hours to reach 5% red rust