Advice on galvanizing
Corrosion resistance of hot dip galvanized steel
To predict the corrosion rate of the zinc layer and therefore the duration of the protection offered by galvanizing, reference is made to the UNI EN ISO 14713 – 1999 standard which provides indications on the average annual loss of thickness of the coating, after identification of corrosiveness, or rather of the aggressiveness of the environment (according to ISO 9223). & nbsp; It is immersion in sea water in European temperate regions. These conditions for zinc are less aggressive than in tropical marine waters, where the corrosion rate is higher. magnitude of the corrosion rate of the coating layer. normal conditions, i.e. in the presence of relative humidity of approximately 70% or higher, i.e. in the presence of relative humidity of approximately 70% or higher, i.e. in the presence of SO2 concentration. The contribution of other pollutants, such as salts, NOx, CO etc. is relevant. significantly higher than the current ones. It follows that, following the indications of UNI EN ISO 14713, the anti-corrosion designer is fully protected. the application of national laws and community directives.
As early as 1994, controls in Europe normally restricted the SO2 concentration to below 60μg / m3. Under such conditions, there is a loss of thickness of the zinc coating of 1.5 – 4μm / year, in the worst conditions. For an average coating of 85μm, the indicative duration of protection, without maintenance requirements, was of the order of 20 – 50 years in the most common atmospheres of European countries and cities. Recent studies which appear that today the concentration of SO2 has been reduced by more than 50% on average, to the figure found in 1991. & With the Millennium Map, a previous Zinc campaign up to 2000 extensively on the territory of Great Britain has obtained empirical evidence of a reduction in the average speed of about 60% of zinc corrosion compared to the values of the early 1990s.
All this means that, with the protection offered by a galvanized coating with a standard thickness of 85 μm, there are no maintenance requirements for the entire useful life of the product, in most applications. The galvanizing obtained on the structural steel profiles usually exceeds the minimum thickness required by the standard. This determines a protection that exceeds 100 years in many environments.In any case, even accepting as a precaution the duration of tab. 3.1 the properties of the zinc coating, achieves a very long-lasting protection: for example, in a coastal area (with appreciable urban pollution) a coating of 100μm carries out its action for approximately 25 years, well beyond the duration of any anti-rust or painting. In fact, according to the reference standards for steel constructions, the forecasts of duration at the first maintenance of polymeric anti-corrosive systems and paints generally do not exceed 10 years.
The protection offered to steel is very long-lasting thanks to the fact that in the atmosphere, zinc forms a very compact and stable protective layer on its surface, consisting of oxides and carbonates (or even hydrated sulphates, depending on the environment). Although very thin, this layer is resistant to aggressive species and is able to bring the corrosion of zinc to a value approximately equal to 1/17 – 1/18 of the speed with which unprotected steel dissolves. & Nbsp; The reaction formation of the first oxide layer begins immediately after the extraction of the piece from the molten zinc bath (with the formation of the first layer of ZnO). The subsequent reactions of surface conversion by carbon dioxide – CO2, occur more or less quickly, with the formation of a mixture of compounds that from local conditions. When the surface gets wet due to rain or condensation, zinc hydroxide Zn (OH) 2 is formed on the zinc surface, the surface which in air is transformed, by interaction with CO2, into basic zinc carbonate – 2ZnCO3 · Zn (OH) 2 is insoluble and protective, which is the so-called conversion layer. As this layer thickens, you speed it up the corrosion (or dissolution) of the zinc plating decreases until it becomes really very low, almost negligible. Hot dip galvanizing thus offers the extremely effective protection that anyone can verify in normal practice.
The passivation layer is strong and cohesive. Once developed so as to cover the entire surface, it prevents the further attack of the metal by oxidizing substances. acidity, but there are many other factors such as the temperature, the composition of the condensation water, the quantity and nature of the pollutants.
The duration of the galvanizing coating is therefore influenced by the presence in the atmosphere of compounds that act on the pH of the surface condensate, which, that is, intervene to determine acidification of the exposure environment. This is the case of atmospheric sulfur dioxide – SO which, upon entering the solution, gives rise to an acid environment, in which the carbonates and basic oxides of the passivation layer are transformed into zinc salts, such as sulphate, incoherent and permeable. to the attack of aggressive species. This allows the corrosion of the underlying metallic zinc layer, with different rates depending on the pollutant concentration. & Nbsp; In marine areas, the corrosion rate of the zinc layer is also significantly influenced by the chloride content, while in urban areas the presence of NOx compounds (nitrogen oxides) and CO, produced by the combustion of fuels, is influential, even if less effectively. & nbsp; The deposition of soot and dust can also be harmful both for the very nature of these substances (mostly carbon in the form of carbon black and salts), both because they can increase the risk of condensation on the surface and retain a greater amount of water (hygroscopic properties).
Dependence on pH
The life of the zinc coating is determined by the stability conditions of the surface conversion layer of the zinc coating. The pH is the variable that determines the continuation of the protection. Below pH values equal to 5.5, the corrosion rate of zinc is high. carbonates dissolve quickly. & nbsp; Alkaline substances are tolerated to a greater extent by zinc coatings, which resist well up to rather high pH values (up to a limit of approximately pH = 12.5).
As already stated, the main types of corrosion for zinc can be divided, based on the environment in which they are generated, into urban, industrial, marine and rural corrosion, with a classification that essentially depends on the SO2 content and of Cl- (in the case of marine atmospheres).
The rural atmosphere is ideal for galvanizing. The low presence of pollutants makes the conversion layer very stable, which is thicker and more protective, increasing over time. This causes the corrosion rate to progressively decrease. Galvanizing, therefore, is an excellent protection for farm buildings, various equipment and accessories and does not require special precautions.
Precautions should be taken only in the case of contact with organic sewage and with acidified irrigation systems. In these conditions, it is necessary to carry out a separation of the galvanization from these environments through protective sheaths of polymeric or bituminous organic material. Animal manure in farms or composting plants is, in fact, very aggressive due to their acidity. In this case, it is advisable to paint only the part of the wall in contact with the organic residues. Another risky situation may arise in greenhouses where irrigation is carried out with water with a pH lower than 5.5, which damages the zinc coating. Even in the countryside, it is necessary to pay attention to the possible pitfalls of corrosion in the soil which also constitutes an interesting corrosive environment, as we will see below.
Highly industrialized areas and urban centers
In areas with high industrial concentration and in urban centers, the air is contaminated with numerous pollutants, the most dangerous for zinc corrosion are sulfur compounds. In humid atmospheres, sulfur dioxide – SO2 produces sulfur acids on the zinc surface. These acids react with the oxide, hydroxide and basic zinc carbonate film, causing the transformation of the conversion into water-soluble salts (as in the case of sulphates), which have poor adhesion with the substrate. In this way, the layer is partially destroyed and the corrosion products form a less stable and cohesive coating, which tends to form indefinitely, at the expense of the underlying zinc metal layers. In other words, the zinc loses the condition of surface passivation and the progressive progress of corrosion is determined. However, the high thicknesses of hot dip galvanizing can guarantee protection for a duration equal to the useful life of the product. In any case, it is necessary to seriously evaluate the performance of the alternative protections, which can reach prohibitive costs when they are required to have longer durations than those that would be offered by galvanizing. Systems such as painting alone may even prove to be less reliable in the field, because they are more susceptible to defects, while in most cases it is possible to choose duplex, galvanizing and varnishing systems, specifically designed to withstand the conditions of that particular environment.
In aggressive atmospheres the corrosion rate tends to be linear with time, so very short observations (of the order of a year or less) make it possible to predict a reliable duration even in the long term. long-lasting hot dip galvanizing applications even in industrial environments. The compatibility judgment must be made on a case-by-case basis. The corrosion rate of zinc can vary between 2 and 8μm / year in the most severe cases. The comparison between performance and maintenance costs of other protective systems may suggest the use of zinc plating, based on the expected service life (ie the time span in which the work will operate) which can be several decades. It is therefore not surprising that the protective action of zinc may be the best possible choice even in these very aggressive environments.
Marine – coastal environments
In marine-coastal environments, the resistance of galvanizing is influenced by the presence of sodium chloride – NaCl, in the form of saline solution suspended in the air. zinc corrosive effects much less relevant than those seen for sulfur oxides. The chlorides, in fact, attack the galvanized surface in a much milder way than they do with steel. & Nbsp; The salt content in the atmosphere decreases very rapidly as you move from the coast to the innermost areas. The situation is also improved by the fact that in the air and water of more temperate climates there is a certain amount of magnesium salts which help to inhibit zinc corrosion. of a duplex system, if you want protections of very long duration.
In any case, in the coastal marine areas the effect of the wind must be considered, which determines a considerable abrasive action. For paints, this can lead to the wear of the polymeric layer in more or less short times. In some cases, the choice of the designers is favorable to the adoption of galvanizing due to its surface hardness, as was the case with the Brooklyn Bridge. Its cables were hot-dip galvanized to resist the erosive action of the wind. & Nbsp; Galvanizing has guaranteed efficient protection since 1883, when the bridge was inaugurated.
Experimental evidence has shown that the effects of chlorides are particularly evident in combination with the acidifying action of sulfur oxides, as often occurs in areas with a high density of industrial settlements near the sea. & nbsp; Due to the large number of factors which contribute to the development of corrosion, it becomes impossible to define a general formula to determine the speed of this process. & nbsp; The choice of the specific value of the corrosion rate within the indicated range, must be made case by case, evaluating the corrosivity of the atmosphere based on the known conditions.
In formulating such evaluations, it is often considered that the atmosphere has the same degree of aggressiveness towards steel and zinc. This approximation, however, appears incorrect, since the corrosion speed of steel as the levels of sulfur and chloride in the atmosphere and humidity increase, has a different trend compared to zinc, whose corrosion proceeds much more slowly, as previously mentioned. & nbsp; Swedish data provides a reference. Unfortunately from you corresponding to those of tab. 3.3. detected in Italy are not yet available, although it is possible to believe that the improvement of the general environmental conditions is accompanied by a more advantageous global situation than the Swedish reference, especially in the most unfavorable conditions, relating to urban and industrial areas.
Corrosion in water
In liquids, even more than in the atmosphere, the pH value is decisive. Other factors also affect the corrosion of zinc in water, such as chemical composition, temperature, pressure, flow rate, agitation and dissolved oxygen concentration. Fresh waters containing mineral salts or calcium and magnesium (hard waters) are not very aggressive. If, on the other hand, the zinc surface remains for a certain time in contact with water with a low content of mineral elements, or when the aeration and, therefore, the presence of CO2 is insufficient, the anti-corrosive layers cannot form. The result is a higher corrosion rate. & Nbsp; Distilled (with minimal presence of salts) and condensed waters, while they should have a pH 7 (neutral), often, collecting the carbon dioxide of the air, acidify passing to a pH around 5, highly corrosive for the galvanized surface.
The corrosive effects of condensation can easily be tested by partially filling a galvanized steel tank with hard water. Once closed, condensation will occur in the space where the air is present. A thin whitish porous layer will form on the upper part of the tank walls rather quickly, while the immersed part will not suffer any damage. This is due to the fact that in the condensate there are no protective salts and, moreover, there is a greater contact with oxygen. between zinc and steel. This corresponds to an inversion of the protection whereby the damaged coating areas lead the corresponding steel areas (with a very limited surface) to be discovered with the role of small anodic areas in the middle of the large cathodic areas of the galvanized coating. Under these conditions, corrosion becomes very fast due to the high values of the current densities involved.
However, even if the phenomenon involves a localization of corrosion, there is no real risk of pitting or pitting. Similar phenomena can occur for fresh water, in the presence of temperatures around 70 ° C. & Nbsp; For the conditions of localized corrosion risk, see the UNI EN 12502-3: 2005 standard.
In general, if the temperature is above 55 ° C, the corrosion products that are formed on the surface have a high particle size and, therefore, show poor adhesion with the zinc coating. & nbsp; As mentioned, also the flow velocity has a certain influence: if it is greater than 0.5m / s, it hinders the formation of the protective layer on the galvanized surface and corrosion occurs faster.
Corrosion in the soil
Under particular conditions, even soils can be corrosive to some extent. The corrosivity of the soil is due both to physical factors (temperature, water absorption and permeability for oxygen) and to chemical factors (concentration of salts, calcium bicarbonate and different pH values from 3 to 9.5). & Nbsp Due to its structure, the soil has a different permeability to air and humidity. Generally the concentration of oxygen is lower than the air, as opposed to that of carbon dioxide which is higher. emerges from the ground. Between inside and outside, the diversity of concentrations of reactive species (especially oxygen) triggers the corrosive pile (for differentiated ventilation).
The soil can also contain pollutants, the presence of which is caused by atmospheric conditions, such as salts, acids, alkalis, mixtures of organic compounds, hydrogen, methane. It can harbor oxidizing or reducing fungi and microorganisms, capable of causing biological corrosion. Consequently, the possible causes of metal degradation in the soil are remarkably complex and the variations in the physical and chemical characteristics between different locations, even close to each other, can be very significant. A measure of the aggressiveness of the soil is given by the resistivity which indicates the concentration of salts and the presence of moisture. The lower the resistivity of the soil, the greater its corrosivity. Furthermore, the more conductive soils are more exposed to the risk of stray currents. Also galvanized steel must be protected from stray currents through the use of known techniques, such as the adoption of insulating joints.
Particularly important is the cathodic protection offered by zinc in this environment. In general, it is considered that the corrosion rate of zinc in the soil is rather low, with average values around 5μm / year. This corrosion can be delayed by protecting the galvanized surface with polymer coatings, bituminous sheaths or any compatible material that results in insulation. This is critical for the safety of posts, pillars and structural elements for the foundations of any metal construction. In this way, the life of the totally underground constructions is extended and the duration of the underground parts of the building is standardized to that of the aerial parts. The aging of organic materials can be effectively counteracted by the action of the underlying zinc coating.
Contact with other metals:
As stated in the chapter on corrosion, contact with other metals can also accelerate corrosion. Zinc is almost always more electronegative than the metals commonly used, therefore it tends to behave as an anode, corroding itself. It is therefore advisable, in the joints with other metals, especially with copper and brass, to insulate the elements with rubber or plastic. & Nbsp; The risk of corrosion of the zinc coatings in case of contact with copper elements is very high. Therefore, any contact should also be avoided, even with water rich in copper ions. & Nbsp; If exposed to air, galvanized structures can remain in contact with stainless steel and aluminum. Instead, it is advisable to isolate them if they are to be immersed in water.
Corrosion in concrete and protection of the rod with zinc plating A particular corrosion environment is concrete. The protection of steel reinforcements from corrosion is a matter of primary importance to prevent structural problems in real estate, too often neglected in the past or hastily resolved by oversizing the rod used and applying an extra thickness of concrete as a protective barrier, called over-iron. P >
Corrosion of concrete reinforcement
In the pores of the newly laid concrete there is a solution of calcium, sodium and potassium hydroxide at pH 13 ÷ 13.8. In these conditions, the steel is covered with a protective film and is perfectly passivated. The corrosion rate is therefore practically zero. Unfortunately, concrete tends to lose these characteristics over time due to the action of carbon dioxide in the atmosphere, which tends to diffuse into the concrete itself, progressively penetrating it (carbonation) up to the reinforcement and causing a drastic reduction in pH (pH9) on site. These changes in the environment destroy the protective oxide layer. At the same time, oxygen diffusion occurs and the corrosion process is triggered in the presence of humidity.
Other acid pollutants can also lead to a change in the pH of the cement, but the action of carbon dioxide is by far the most prevalent. The chlorides deserve a separate discussion, which cause attacks even at low concentrations, dangerous because they tend to be localized and punctual. To these phenomena it is necessary to add the cases in which corrosion is favored by the presence of deep cracks up to the reinforcements, often the cause of insufficient curing (spraying with water in the days immediately following the casting), high porosity caused by unsuitable cement / water ratios, inclusions sand or particularly aggressive pollutants present in the atmosphere.
Protection of the rod with zinc plating
Hot dip galvanizing of rebar is an effective preventive measure, since zinc remains passive even in carbonanate concrete. When fully operational, the corrosion rate is less than 1µm / year even in alkaline concrete, but becomes significant when the chloride content exceeds 1%. The steel of the rods corrodes with a certain speed in conditions of direct exposure to marine spray for coastal constructions or for bridges when spreading salt, a technique in use to avoid the formation of ice on the roadways in the winter season.
Experimental tests have shown that the ternic treatment associated with hot dip galvanizing does not cause changes in the properties mmechanical characteristics of the steel, for which the same characteristic parameters of the basic steel apply to the design for galvanized steel. In addition, the adhesion of the galvanized steel rod to the concrete is greater than that between steel and concrete. For carbonated concrete, a zinc coating of the order of 80 – 100µm determines an increase in durability of 50 – 70 years compared to unprotected steel reinforcement. In the presence of chloride contamination, with hot dip galvanizing it is possible, however, to obtain a doubling of the useful life of the product.
The zinc layer is passive by reaction with the strongly alkaline cement. The carbonation of the cement at the threshold value for the propagation of corrosion in uncoated steel – see part a) – does not damage the passivation of the zinc which, therefore, continues to protect the steel until the threshold value is reached -zinc – see part b) – essentially due to the accumulation of chlorides. Until the steel threshold value is reached (corresponding to the total consumption of zinc), the steel remains intact and the concrete does not show cracking or detachment.
Corrosion due to contact with other materials
Green wood should not be placed in contact with galvanized steel, since some substances present in it are able to corrode the zinc. Similarly, hot dip galvanized nails should not be used in water regardless of whether the wood is impregnated or not. & nbsp; Other dry materials, such as rock wool, are not aggressive to zinc.
Gypsum attacks zinc, but it is a very corrosive material for all metallic materials, even when it is dry. In any case, plaster and galvanized steel must not be put into contact if long durations are required. & Nbsp; Zinc reacts strongly in contact with alkaline mortars. In a short time (in the order of weeks) due to exposure to calcium hydroxide, zinc gives rise to the formation of calcium hydroxyzincate – Ca · Zn (OH) 3 · 2H2O which has protective effects for galvanizing. Corrosion behavior is, therefore, much better than unprotected steel. & Nbsp; In Portland and pozzolanic cement, galvanizing is able to significantly extend the life of the products if used to protect the reinforcements. However, it is not suitable for contact with high magnesium-content cements (cast or quick-setting cement) or building materials with pH above 12.5. Galvanized steel, while behaving much better than black steel, cannot be exposed to direct contact especially if in continuous humidity conditions. In these cases, it is necessary to protect the galvanized surface with sheaths or other waterproof material.
Among the different types of corrosion, the so-called white rust deserves further attention, consisting mainly of zinc hydroxide and minimally of oxide and carbonate. & nbsp; It is an oxidation process that mainly affects recently galvanized surfaces, since no protective layer has yet formed on them. It comes in a recognizable white-powdery form and is usually a consequence of incorrect storage or incorrect packaging of the material. It develops if two parallel galvanized surfaces facing each other are brought into contact with the formation of a cavity filled with condensation water. same conformation of the system. This determines a galvanic pile due to the difference in aeration (of oxygen concentration). & Nbsp; Under these conditions, corrosion proceeds very quickly, with the formation of voluminous and incoherent corrosion products (hydroxycarbonate, oxide and zinc hydroxide). P >
The periods of the year that most favor the onset of white rust are autumn and winter, due to the high humidity due to rain and fog, and low temperatures. Equally harmful conditions are caused by sudden drops in temperature. small in size, they create ideal conditions for the formation of white rust.
In fact, in most cases, even if they are very conspicuous, the damage caused by white rust is not substantial, as small portions of zinc produce large quantities of white, dusty and very noticeable rust. In geblack, the control with magnetic thickness gauge detects, in the great majority of cases, small variations in the galvanizing thickness. Furthermore, if the triggering causes persist, minimal percentages of white rust transform into a completely harmless protective layer, which can possibly cause problems during subsequent painting. & Nbsp; Only if the humidity is very intense and persists for prolonged times it can cause considerable damage, with local destruction of the zinc coating. To evaluate the actual damage it is advisable to completely remove the corrosion products and measure the thickness of the residual coating. If this test verifies the presence of thickness values that still comply with the regulations, it may be sufficient to carefully remove the whitish layer. Even if locally the galvanizing, once the layer of white rust has been removed, appears darker and opaque, the corrosion protection properties are not at all affected and the phenomenon cannot be the cause of rejection or contestation of the material, provided that the thickness is higher than the minimums established by the standards or specifications. If the coating no longer has a consistent thickness, it is necessary to restore the protection in accordance with the regulations.
To prevent the formation of white rust it is sufficient to adopt simple precautionary measures during the storage and transport of galvanized products. First of all, it is necessary not to deposit recently galvanized parts for a long time, outdoors, in highly humid atmospheres, in tall grass, in puddles and mud. Instead, it is advisable to place the steel parts in layers interspersed with spacers (for example wooden squares) at a distance of about 15cm from the ground, arranging them in such a way as to prevent water from collecting inside the artifacts. It is also advisable to prevent the contact points from affecting the entire surface, and the flow of water should be favored by creating slight differences in level. Finally, it is advisable not to cover the galvanized elements with plastic sheets or laminates, under which humid air could easily stagnate and condensate to form.
During transport, it is necessary to ensure sufficient ventilation and avoid the collection of moisture. & nbsp; In particular, in transport by sea it is necessary to provide for the adoption of chemical protection measures. The use of passivating or surface sealants is recommended. Contact with other transported substances, or their residues, if aggressive, should be avoided.