Technical Info Sheet #24
Galvanized steel is not only found above ground – some of it is underground. The conditions there are completely different from those above ground level, where structural parts are in the open air. To give you an idea of what goes on underground, we have put together this information sheet.
GALVANIZED STEEL IN THE ATMOSPHERE
For over 150 years, galvanized steel has been used and exposed to the elements. It all started with the galvanizing of tubs, watering cans, buckets, and tools; later also steel structures, traffic sign posts, guide rails, high tension pylons, etc.
Because of the diversity of applications, there is an excellent picture of how zinc behaves under atmospheric conditions.
The lifetime of the zinc coating as protection for the underlying
steel is defined in the international standards ISO 9223, ISO 9224 and ISO 14713-1. Also, to calculate the expected period until the first maintenance, a special tool can be used; the Zinc Coating Life Predictor, which can be found on our website. However, gates, lampposts, fence posts and pipes, in addition to being placed in the atmosphere, are also placed at least partially in the ground or soil.
THE ZINC LAYER RECEIVES AN ADDITIONAL BESCHEMLAYER
Immediately after hot-dip galvanizing, the surface of the still fresh zinc layer will quickly react with oxygen to form zinc oxides. Zinc is a relatively base metal and, like many other metals, will want to return to its lowest energy level, that of oxide.
The protection against corrosion of the zinc coating begins only after a few days with the formation of a so-called zinc patina layer through the action of CO2 from the atmosphere. The formation of the patina layer is clearly visible. The initially shiny zinc layer gradually becomes duller and grayer. The formation of this important additional protective layer takes up to 12 months after galvanizing and depends on the relative humidity of the air.
The thickness of this patina layer is less than ¼ micrometer. In any case, wait at least several weeks before placing the galvanized steel in the ground so that there is some evidence of a formed zinc patina layer.
LAND APPLICATION
As we can observe in daily practice, a lot of galvanized steel is used in the soil without significant premature corrosion. Very occasionally, however, deterioration of the zinc layer can be observed after only a few years. This incipient deterioration of the zinc is a signal of an aggressive condition specifically in this situation. The corrosion rate of galvanized steel in soil can vary from less than 0.2 microns per year in favorable conditions, to 20 microns per year or more in very aggressive soil types.
There are several standards dealing with the probability of corrosion when placed in the soil. In Europe we know EN 12501 (parts 1 and 2) and for the Netherlands NEN 6766. The German standard DIN 50929-3 is often used for the construction of solar panel parks. It is recommended to review these standards in those situations where further investigation is desired or necessary. If necessary, engage a geotechnical consulting firm to help you with this.
COMPOSITION OF THE SOIL
Soils in the Benelux are very diverse in composition. We all know the basic names such as clay, sand, loam, the so-called minerals. The soil type is determined by the ratio of these materials (see ISO 11277). In addition, there is an organic fraction in the soil, consisting of humus and peat. The combination of the minerals and the organic matter, together with the soil texture, help determine the properties. The texture of the soil is determined by the size of the particles.
In general, galvanized steel will give a good performance in almost all soil types in the Benelux. Placed in relatively acid soils, such as peat soils, hot-dip galvanized steel will often not be able to do without an extra protective layer.
WHAT FACTORS PLAY A ROLE
The main factors determining soil corrosivity are moisture content, acid number (pH), and chlorides. These factors are influenced by additional characteristics such as aeration, temperature, resistivity, and texture. A general rule of thumb is that hot-dip galvanized steel performs well in red, yellow and brown sandy soils and less well in gray, clayey soils. This is because soils with larger particles remove moisture from the surface faster, so the galvanized steel is less exposed to moisture. Also, as a result, aeration of the galvanized coating is better. Because zinc is an amphoteric metal, it dissolves easily in acidic and alkaline conditions. At a ground temperature of 4℃, corrosion is half as much as at 20℃.
PRACTICE AS A GUIDE
Soil composition can be determined by an Environmental Laboratory after taking a soil sample. What determines the degree of corrosion for a zinc layer are all the factors that can cause zinc to corrode. The chloride content of soil samples, along with the moisture content, is an important factor in estimating the risks on site. The presence of oxygen is a determining factor, and acidity expressed as pH also plays an important role. In addition, sulfate and sulfur affect the durability of the galvanized steel. What can be determined is the (electrical) resistance (or in other words, degree of conductivity) of the soil. The higher the resistance the more favorable it is for the life expectancy of galvanized steel. When placed in groundwater, the degree of groundwater displacement is influential.
In short: a whole range of factors play a role. For this reason, it is not possible to simply determine in advance whether the zinc layer is losing its protective function for the steel at an accelerated rate. It is therefore advisable to visit the location in question beforehand. In doing so, check whether galvanized steel may have been previously placed in the ground at that location. By exposing a piece of it, it is easy to assess whether any deterioration can be seen. Together with data on when the part in question was placed, a reasonably useful prediction can be made.
CORROSION VISIBLE AT TRANSITIONS
There is an increased risk of corrosion damage at the interface between the atmosphere and soil and at the interface between soil and groundwater, as corrosion cells may form on the zinc surface due to the different conditions. Extra attention will be required when there is evidence of such corrosion damage at the location in question. A lawn mower, for example, can easily damage this area of the object, reducing its resistance to corrosion. Typically, this transition area also remains damp for a longer period of time.
HOW TO PREVENT DETERIORATION OF THE ZINC?
If you have doubts about the suitability of the soil and do not want to or cannot take a risk, then the application of an additional paint system can be considered. A bituminous paint that is applied in a fairly thick layer or in two coats is often used for this purpose. A thick barrier is created between the soil and the galvanized steel by this bitumen. The agent has no anti-corrosion properties. It is applied hot or with solvent in it. It can withstand acids but not solvents.
It is also important that no moisture or water can accumulate around the galvanized parts in the ground. In other words, water should not be allowed to stand, so that a pool is not created.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance
EN ISO 9223
Corrosion of metals and alloys – Atmospheric corrosion rate – Classification,determination and estimation
EN ISO 9224
Corrosion of metals and alloys – Atmospheric corrosivity – Guideline values for corrosivity categories
ISO 11277
Determination of particle size distribution in mineral soil materials
len – Method by sieving and sedimentation
EN 12501-1
Protection of metallic materials against corrosion – Corrosion
likelihood in soil – Part 1: General
EN 12501-2
Protection of metallic materials against corrosion – Corrosion
likelihood in soil – Part 2: Low alloyed and non alloyed ferrous materials
NEN 6766
Corrosion of steel elements in the subsurface – Requirements for design and application.
DIN 50929-3
Korrosion der Metalle – Korrosionswahrscheinlichkeit metallener Werkstoffe bei äußerer Korrosionsbelastung – Teil 3: Rohrleitungen und Bauteile in Böden und Wässern
DVGW GW 9:2011
Evaluation of soils in view of their corrosion behavior toward buried pipelines and vessels of non-alloyed iron materials
World Reference Base (WRB)
International standard for soil classification, developed by the International Union of Soil Sciences (IUSS) and the Food and Agriculture Organization (FAO)
ASTM D2487 (Unified Soil Classification System – USCS)
Classifies soils based on grain size, gradation, and plasticity.
EN ISO 14688-1
Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description
EN ISO 14688-2
Geotechnical investigation and testing – Identification and classification of soil – Part 2: Principles for a classification
TECHNICAL DATA SHEET 10
Corrosion resistance of hot-dip galvanized steel
