What makes zinc corrode

Since the carbonates have very low solubility in water they give excellent protection to the surface of the zinc coating. The original shiny surface with a metallic lustre disappears to be replaced by matt, light grey colour fig. Fig 1. Exposed surface of a zinc coating with outer layer of pure zinc. Discoloured surface on lighting column. Coating consists mainly of an iron-zinc alloy that extends to the surface. Outdoor air contains greater or lesser amount of corrosive elements — gases, soot, humidity fog, dew, rain, snow , inert and aggressive dust.

Levels can vary with location and the time of the year. Sulphates and sulphites of zinc are water soluble and have poor adhesion to the zinc surface. They are therefore washed away easily by rain. A fresh zinc surface is then exposed to attack by oxygen in the air and the corrosion cycle is repeated. Corrosion in air containing sulphur oxides is therefore greater than in clean air.

However, the amount of sulphur dioxide in the atmosphere has decreased drastically during recent years, and consequently zinc corrosion has also decreased. In marine environments, the corrosion of zinc is influenced by the salt content of the air. However, marine air contains small quantities of magnesium salts, with good passivating influences.

Corrosion is therefore not as great as might be expected. The salt content of the air reduces quickly away from the coast. The corrosion of zinc is influenced by many factors. This means that a generally applicable formula for corrosion rates cannot be given. Zinc coatings have, however, been used for a long time, under a wide range of conditions, to protect steel from rust. A large number of long-term tests have also been conducted.

Knowledge about the corrosion of zinc and corrosion rates in different environments is therefore good. Today, there are examples of zinc coatings that have been exposed for more than one hundred years. The colour of corrosion products varies according to the environment in which they are formed.

Marine environments give somewhat whiter corrosion products compared with rural and urban environments. Corrosion products are usually darkest in urban environments. The zinc surface is generally covered with a protective layer of corrosion products when it is submerged in a liquid. However, liquids can be acidic or alkaline and can contain dissolved or solid particles of aggressive substances. The temperature and flow rate of the liquid are also of significance.

All this means that the protective layer can have a highly varying composition or may not form at all. Electro-chemical corrosion, which plays a subordinate role in air, is of greater significance in liquids.

Edited by. Stephen D. Cramer ; Stephen D. This Site. Google Scholar. Bernard S. ASM International. Publication date:. Book Chapter. Zinc nickel plating significantly slows down the corrosion process by keeping moisture from reaching the base metal. Zinc corrodes approximately times slower than other metals, and even if the underlying metal was to become exposed, Zinc still protects it from corrosion.

When in the presence of an electrolyte, Zinc will corrode first and protect the metal is in contact with.

When zinc reacts with oxygen, zinc oxide is formed. When this zinc oxide reacts with water it forms zinc hydroxide. This zinc hydroxide bonds with carbon dioxide to produce a thin layer of zinc carbonate. Environmental conditions that interfere with the formation of such films may attack zinc quite rapidly. The important factors that control the rate at zinc corrodes in atmospheric exposures are: The duration and frequency of moisture The rate at which the surface dries The extent of industrial pollution of the atmosphere.

In dry air, zinc is slowly attacked by atmospheric oxygen. A thin, dense layer of oxides formed on the surface of the zinc, and outer layer then forms on top of it. Although outer layer breaks away occasionally, the under layer remains and protects the metal restricting its interaction with the oxygen. Under these conditions, which occur in some tropical climates, the zinc oxidizes very slowly.

The rate of drying is also an important factor because a thin moisture film with higher oxygen concentration promotes corrosion.

Deterioration in the atmosphere is sometimes called weathering. This definition encompasses a great variety of environments of differing corrosivities. The factors that determine the corrosivity of an atmosphere include industrial pollution, marine pollution, humidity, temperature especially the spread between daily highs and lows that influence condensation and evaporation of moisture and rainfall.

The atmosphere, as far as corrosion is concerned, is not a simple invariant environment. The influence of these factors on the corrosion of zinc is related to their effect on the initiation and growth of protective films.

Corrosion of Zinc in Water. The corrosion of zinc in water is largely controlled by the impurities present in the water. Naturally occurring waters are seldom pure. Even rainwater, which is distilled by nature, contains nitrogen, oxygen, CO 2 , and other gases, as well as entrained dust and smoke particles. Water that runs over the ground carries with it eroded soil, decaying vegetation, living microorganisms, dissolved salts, and colloidal and suspended matter.

Water that seeps through soil contains dissolved CO 2 and becomes acidic. Groundwater also contains salts of calcium, magnesium, iron, and manganese.

Seawater contains many of these salts in addition to its high NaCl content. All of these foreign substances in natural waters affect the structure and composition of the resulting films and corrosion products on the surface, which in turn control the corrosion of zinc.

In addition to these substances, such factors as pH, time of exposure, temperature, motion, and fluid agitation influence the aqueous corrosion of zinc. As in the atmosphere, the corrosion resistance of a zinc coating in water depends on its initial ability to form a protective layer by reacting with the environment.

In distilled water, which cannot form a protective scale to reduce the access of oxygen to the zinc surface, the attack is more severe than in most types of domestic or river water, which do contain some scale-forming salts. The scale-forming ability of water depends principally on three factors: the hydrogen ion concentration pH value , the total calcium content and the total alkalinity.

If the pH value is below that at which the water would be in equilibrium with calcium carbonate CaCO 3 , the water will tend to dissolve rather then to deposit scale.