Page 190 - CW E-Magazine (13-8-2024)

Special Report

working load has to be reduced, and the to the top, and consist of alloys of iron Both pickling and passivation are
system derated properly (safety factors and aluminium, while dross – which chemical treatments applied to the
need to be increased). Ropes will also sinks to the bottom – is made of iron surface of SS to remove contami-
need frequent inspection, and the use and zinc. Accumulation of these wastes nants and assist the formation of a
of galvanized ropes for heavy-duty hot interferes with galvanization, and continuous chromium-oxide, passive
metal cranes should be avoided. hence it is removed and taken off-site. film. Surfaces must be free of grease
It is possible to recycle most of the and dirt before using these acid
Sulphur dioxide in the air, the metallic waste products. Air pollution that treatments.
natural acidity of rain, and other reac- occurs in the heating of a galvanization
tive substances may reduce the life of tank may also be a concern. Safety and SS resists corrosion best if they are
galvanized steels. The high conducti- health issues related to the hazards of clean and smooth. Clean means being
vity of seawater also increases the risk molten hot metal and pickling tank free of contaminants on or in the sur-
of corrosion by converting the coated operation may also be issues with this face that can either reacts with the steel
zinc to water-soluble zinc chloride. corrosion prevention method(11). (like carbon steel or salt) or that create
Zinc-coated car frames corrode in the crevices or other initiation points where
winter season as a result of road salt. Inspection of galvanized surfaces corrosion can start. Smooth means hav-
Additional protection from a sacrificial When it comes to inspecting gal- ing a low surface profile at the ‘micro’
anode or a paint application needs to be vanized surfaces, automated surface level. Mechanically abrading the sur-
considered in such cases. inspection systems are adopted for face can roughen the steel’s surface and
the online surface quality monitoring may also embed unwanted particles.
Galvanization: Sustainability and of galvanized metals, and for classi- After mechanical cleaning, passivation
recycling fying and detecting surface area defects is required for maximum corrosion
Unlike the coatings that use volatile in continuous galvanization processes. resistance.
organic compounds (VOCs), galva- In the case of batch production, visual
nizing is considered environmentally- inspection gives fairly accurate results. The common feature of chemical
friendly and sustainable. Gibbs Free Zinc does not form a coating on treatments is that they all clean the sur-
Energy (GFE) is one of the measures unclean surfaces. face of the steel. They may also smooth
of ranking the sustainability of metals. or roughen the steel surface or leave it
It represents the energy used from Coating thickness is a critical para- unaffected depending on which process
the mining to the metal stage in MJ/ meter, as the life of the coating is pro- is chosen. But if carried out properly,
Kg. Zinc has a GFE value of 3.0-MJ/ portional to coating thickness and its they all increase the corrosion resis-
Kg, whereas steel has a GFE value of adherence (bonding) to the substrates. tance.
6.5-MJ/Kg. Zinc is more sustainable Surface finish and the decorative quality
than steel, and the zinc requirement is are also checked against the speci- What is passivation?
around 5% of the mass of the steel it fied requirements. Because the molten Passivation is the loss of chemical
protects. zinc cannot penetrate small cavities, a reactivity experienced by certain met-
coating discontinuity will show up as als and metal alloys under particular
There are opportunities in the a bleeding or a blowout. In the case of environmental conditions. The metals
recycling of waste products produced reactive steels, the flaking of the surface or alloys become inert and act as if they
in galvanizing, which reduce pollution coating may be observed. Contami- are noble metals (e.g., platinum and
and ensure adherence to local environ- nants on the surface may also show up gold). The following metals or metal
mental compliance rules. The pickling, as a surface defect. Root-cause analysis alloys containing them exhibit passivity:
rinsing and cleaning baths of the galva- of surface defects could lead to appro- iron, nickel, silicon, chromium, and
nization process, in particular, provide priate preventive measures. titanium. Under typical conditions, zinc,
opportunities for recycling wastes. cadmium, tin, uranium, and thallium
Acids and other chemicals need to be Passivation chemical surface treatments only show a limited passivity. Although
disposed of as per regulation. Chemical treatments are useful passivity is often difficult to define, it
tools in cost effectively achieving peak is often described quantitatively by
Some quantity of metallic waste is performance with stainless steel (SS). characterizing the behaviour of metals
generated in a galvanizing tank as dross With appropriate training, hazards asso- that exhibits this unusual effect. Quanti-
as well as skimmings. Skimmings float ciated with their use can be managed. fication involves Tafel analysis (Tafel

190 Chemical Weekly August 13, 2024

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