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Jaideep Greer
Jaideep Greer

Steelwork Corrosion Control Ed 2

In many instances, steelwork will be in a warm dry interior where it will not corrode, and the structural stability of the building will not be threatened during its design life (generally taken as 50 years). In such conditions (classified as C1) no corrosion coating is required. Examples include steelwork inside dry buildings with neutral atmospheres such as multi-storey office buildings, shops, schools, hotels, residential buildings, airport terminals, and hospitals, etc.

Steelwork Corrosion Control Ed 2

However, when steelwork is exposed to moisture, corrosion will occur at a rate depending on the severity of the environment. In such cases, a coating system appropriate to the environment category should be provided. Note that some buildings may contain areas where different environment conditions apply e.g. hospitals would normally be C1, but may contain kitchens and laundry areas that would normally be C3. Some types of buildings, such as car parks may fall into any of the above categories or combinations of them depending upon their location, design and construction. Higher risk categories for interior environments (e.g. C4 and C5) are not covered in this article and users are advised to seek specialist advice if their project involves such situations.

The period of reasonable freedom from severe corrosion of the steelwork that might lead to weakening of the structure. This figure assumes no mechanical damage in service that no maintenance is carried out and that up to 1mm of steel may be lost from the surface at the corrosion rate for each environment given in BS EN ISO 12944-2[1]. Visible steelwork will normally be accessible for maintenance and if repainting is carried out the quoted structure life will be extended.

The expected period to maintenance of the protective coating. More frequent re-coating may often be preferred for decorative reasons because of fading, contamination, wear and tear, etc. Hidden steelwork is assumed to be not accessible for maintenance, thus a figure for coating life of hidden steelwork systems is not applicable.

Coating system durability given in the following tables is based on practical experience. It is the expected life, in years, before first major maintenance. This is taken as degradation level Ri3 from BS EN ISO 4628-3[6] (1% of surface area rusted). It should be noted that this does not imply a guarantee of life expectancy. The durability of galvanized steelwork is derived from the figures in BS EN ISO 14713[7].

Brickwork cladding or other masonry, can develop cracks and leakage over time. When steelwork is in contact with, or embedded in a brick/masonry outer leaf, one of the following systems should be used. In some regions, stainless steel may be required for embedded members by local regulations or be deemed necessary to provide adequate durability. Specialist advice should be sought.

Engineers on major building projects continue to echo the sentiment that "painting amounts to 10% of the job, but provides 90% of the problems". This second edition of Steelwork Corrosion Control provides sound advice and authoritative guidance on the principles involved and methods of achieving sound steel protection. Taking into account the considerable developments in the paint protection industry, Steelwork Corrosion Control has been comprehensively updated to include new materials and coating systems, and the number of new ISO / BS / European standards and codes of practice on paints and painting, health and safety, and environmental issues. It is a must-have guide for engineers, architects and designers for whom the protection of structural steelwork is an important, albeit relatively minor, part of their professional activities. David Deacon is the President Elect of the Institute of Corrosion and a Fellow of FTCS (Fellowship of Technical Service Coating). Derek Bayliss is a Past President of the Institute of Corrosion and has served as Chairman of BS 5493 (concerned with coating strucures against corrosion).

The lead author, Derek Bayliss, is a well-established name in the field. He was Past President of the Institution of Corrosion Science and Technology, and has served as Chairman of BS 5493 (concerned with coating structures against corrosion). He is widely known as a lecturer on corrosion prevention, coating and surface preparation topics.David Deacon FICorr FTSC is the Director of the UK's Steel Protection Consultancy. He qualified as a paint technologist in 1964 and worked with British Aluminium, the British Iron & Steel Research Association, the Castoral Burmah Group and the Albright & Wilson Group. He set up as a consultant in 1972 and has been working in this capacity for the past 30 years on a range of projects worldwide.

'It describes recent developments, and the materials and processes used, including coverage of standards, codes of practice and other publications useful in quality control.' - World Surface Coatings Abstracts'...the textbook provides a good overview of steelwork corrosion control' - E-Streams'This handbook style textbook is strongly recommended for university libraries and community college libraries supporting engineering technology curricula.' - E-Streams

The corrosion of structural steel is an electrochemical process that requires the simultaneous presence of moisture and oxygen. Essentially, the iron in the steel is oxidised to produce rust, which occupies approximately six times the volume of the original material. The rate at which the corrosion process progresses depends on a number of factors, but principally the 'micro-climate' immediately surrounding the structure.

The corrosion of steel can be considered as an electrochemical process that occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution. Electrons are released from the anode and move through the metallic structure to the adjacent cathodic sites on the surface, where they combine with oxygen and water to form hydroxyl ions. These react with the ferrous ions from the anode to produce ferrous hydroxide, which itself is further oxidised in air to produce hydrated ferric oxide (i.e. red rust.) The sum of these reactions can be represented by the following equation:

However, after a period of time, polarisation effects such as the growth of corrosion products on the surface cause the corrosion process to be stifled. New, reactive anodic sites may be formed thereby allowing further corrosion. In this case, over long periods, the loss of metal is reasonably uniform over the surface, and this is usually described as 'general corrosion'. A schematic representation of the corrosion mechanism is shown (above right).

When two dissimilar metals are joined together and in contact with an electrolyte, an electrical current passes between them and corrosion occurs on the anodic metal. Some metals (e.g. stainless steel) cause low alloy structural steel to corrode preferentially whereas other metals (e.g. zinc) corrode preferentially themselves, thereby protecting the low alloy structural steel. The tendency of dissimilar metals to bimetallic corrosion is partly dependent upon their respective positions in the galvanic series. The further apart the two metals in the series the greater the tendency.

Another aspect that influences bimetallic corrosion is the nature of the electrolyte. Bimetallic corrosion is most serious for immersed or buried structures, but in less aggressive environments e.g. stainless steel brick support angles attached to mild steel structural sections, the effect on the steel sections is minimal. No special precautions are required in most practical building or bridge situations. For greater risk situations, gaskets, sleeves and similar electrically insulating materials should be used. Alternatively the application of a suitable paint system over the assembled joint is also effective.

The tendency for bimetallic corrosion is also influenced by the relative surface areas of the cathodic and anodic metals (Ac/Aa). In simple terms, the greater the Ac/Aa ratio, the greater the tendency for bimetallic corrosion.

In some circumstances the attack on the original anodic area is not stifled and continues deep into the metal, forming a corrosion pit. Pitting more often occurs with low alloy structural steels in continually wet conditions or buried in soil rather than those exposed in air. Hence, pitting corrosion is rarely encountered on typical modern steel buildings or bridges.

Crevices can be formed by design detailing, welding, surface debris, etc. Available oxygen in the crevice is quickly used by the corrosion process and, because of limited access, cannot be replaced. The entrance to the crevice becomes cathodic, since it can satisfy the oxygen-demanding cathode reaction. The tip of the crevice becomes a localised anode and high corrosion rates occur at this point.

This is the proportion of total time during which the surface is wet, due to rainfall, condensation etc. It follows, therefore, that for unprotected steel in dry environments e.g. inside heated buildings, corrosion will be minimal due to the low availability of water. The requirement for the application of paints or coatings becomes unnecessary other than for appearance or fire protection purposes.

Within a given local environment, corrosion rates can vary markedly, due to effects of sheltering and prevailing winds etc. It is therefore the 'micro-climate' immediately surrounding the structure, which determines corrosion rates for practical purposes.Because of variations in atmospheric environments, corrosion rate data cannot be generalised. However, environments can be broadly classified, and corresponding measured steel corrosion rates provide a useful indication of likely corrosion rates. More information can be found in BS EN ISO 12944-2[1] and BS EN ISO 9223[2].

Cost effective corrosion protection of structural steelwork should present little difficulty for common applications and environments if the factors that affect durability are recognised at the outset. 041b061a72


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