Corrosion

Corrosion in nonmetals

Most brittleness, such flaws cause a dramatic reduction in the strength of a glass object during its first few hours at room temperature.

The degradation of Plastic shopping bags often do not include these additives so that they break down more easily as litter.

Corrosion of glasses

The corrosion of silicate glasses in aqueous solutions is governed by two mechanisms: pH of contacting solution: the rate of ion exchange decreases with pH as 10^-0.5pH whereas the rate of hydrolytic dissolution increases with pH as 10^{0.5pH [2]}

Numerically corrosion rates of glasses are characterised by normalised corrosion rates of elements NR_i (g/cm² d) which are determined as the ratio of total amount of released species into the water M_i (g) to the water-contacting surface area S (cm²), time of contact t (days) and weight fraction content of the element in the glass f_i:

NR_i=M_i/S·f_i·t

The overall corrosion rate is a sum of contributions from both mechanisms (leaching + dissolution) NR_i=Nrx_i+NRh. Diffusion-controlled leaching (ion exchange) is characteristic of the initial phase of corrosion and involves replacement of alkali ions in the glass by a hydronium (H₃O⁺) ion from the solution. It causes an ion-selective depletion of near surface layers of glasses and gives an inverse square root dependence of corrosion rate with exposure time. The diffusion controlled normalised leaching rate of cations from glasses (g/cm² d) is given by:

NRx_i=2·ρ·(D_i/π·t)^1/2

where t is time, D_i is the i-th cation effective diffusion coefficient (cm²/d), which depends on pH of contacting water as D_i = D_i0·10^-pH, and ρ is the density of the glass (g/cm³).

Glass network dissolution is characteristic of the later phases of corrosion and causes a congruent release of ions into the water solution at a time-independent rate in dilute solutions (g/cm² d):

NRh=ρr_h,

where r_h is the stationary hydrolysis (dissolution) rate of the glass (cm/d). In closed systems the consumption of protons from the aqueous phase increases the pH and causes a fast transition to hydrolysis^[3]. However further silica saturation of solution impedes hydrolysis and causes the glass to return to an ion-exchange, e.g. diffusion-controlled regime of corrosion.

In typical natural conditions normalised corrosion rates of silicate glasses are very low and are of the order of 10^-7 - 10^-5 g/cm² d. The very high durability of silicate glasses in water makes them suitable for hazardous and nuclear waste immobilisation.

Glass corrosion tests

  There exist numerous standardized procedures for measuring the corrosion (also called chemical durability) of glasses in acidic environments, under simulated environmental conditions, in simulated body fluid, at high temperature and pressure^[5], and under other conditions.

In the standard procedure ISO 719^[6] a test of the extraction of water soluble basic compounds under neutral conditions is described: 2 g glass, particle size 300-500 μm, is kept for 60 min in 50 ml de-ionized water of grade 2 at 98°C. 25 ml of the obtained solution is titrated against 0.01 mol/l HCl solution. The volume of HCl needed for neutralization is recorded and classified following the values in the table below.

0.01M HCl needed to neutralize extracted basic oxides, ml	Extracted Na2O equivalent, μg	Hydrolytic class
to 0.1	to 31	1
above 0.1 to 0.2	above 31 to 62	2
above 0.2 to 0.85	above 62 to 264	3
above 0.85 to 2.0	above 264 to 620	4
above 2.0 to 3.5	above 620 to 1085	5
above 3.5	above 1085	>5

The remainder of this article is about electrochemical corrosion.

Electrochemical theory

Main article: Electrochemistry

One way to understand the structure of metals on the basis of particles is to imagine an array of positively-charged ions sitting in a negatively-charged "local thermodynamic equilibrium that can often be described using basic chemistry and a knowledge of initial conditions.

Galvanic series

Main article: Galvanic series

In a given saeeff environment (one standard medium is aerated, room-temperature Galvanic series, and can be a very useful in predicting and understanding corrosion.

Resistance to corrosion

Some metals are more intrinsically resistant to corrosion than others, either due to the fundamental nature of the electrochemical processes involved or due to the details of how reaction products form. For some examples, see galvanic series. If a more susceptible material is used, many techniques can be applied during an item's manufacture and use to protect its materials from damage.

Intrinsic chemistry

  The materials most resistant to corrosion are those for which corrosion is platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth, and is a large part of their intrinsic value. More common "base" metals can only be protected by more temporary means.

Some metals have naturally slow reaction oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.

Passivation

Main article: Passivation

Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as silicon.

These conditions required for passivation are specific to the material. The effect of pH is recorded using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include: high solder can often circumvent passivation mechanisms.

Surface treatments

 

Applied coatings

Main article: Galvanization

steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with a more active metal such as zinc or cadmium.

Reactive coatings

If the environment is controlled (especially in recirculating systems), surfactants (i.e. long-chain organic molecules with ionic end groups).

 

Anodization

Main article: Anodising

Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.

Controlled Permeability Formwork

Main article: Controlled Permeability Formwork

Controlled Permeability Formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement. CPF has been used in environments to combat the effects of frost and abrasion.

Cathodic protection

Main article: Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the electrochemical cell.

It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. niobium coated rod and wires.

Corrosion in passivated materials

rust and other bulk corrosion, they often escape notice and cause problems among those who are not familiar with them.

Pitting corrosion

Main article: Pitting corrosion

Certain conditions, such as low concentrations of oxygen or high concentrations of species such as stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment, which often includes ensuring that the material is exposed to oxygen uniformly (i.e., eliminating crevices).

Weld decay and knifeline attack

Main article: Intergranular corrosion

niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name applies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable¹.

Crevice corrosion

Main article: Crevice corrosion

Crevice corrosion is a corrosion occurring in spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.

Galvanic corrosion

Main article: Galvanic corrosion

galvanic series. For example, zinc is often used as a sacrificial anode for steel structures, such as pipelines or docked naval ships. Galvanic corrosion is of major interest to the marine industry and also anywhere water can contact pipes or metal structures.

Factors such as relative size of anode (smaller is generally less desirable), types of metal, and operating conditions (temperature, humidity, salinity, &c.) will affect galvanic corrosion. The surface area ratio of the anode and cathode will directly affect the corrosion rates of the materials.

Microbial corrosion

Main article: Microbial corrosion

galvanic corrosion.

High temperature corrosion

High temperature corrosion is chemical deterioration of a material (typically a metal) under very high temperature conditions. This non-galvanic form of corrosion can occur when a metal is subject to a high temperature atmosphere containing oxygen, sulphur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.

The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.

Economic impact

The US Federal Highway Administration released a study, entitled Corrosion Costs and Preventive Strategies in the United States, in 2002 on the direct costs associated with metallic corrosion in nearly every U.S. industry sector. The study showed that for 1998 the total annual estimated direct cost of corrosion in the U.S. was approximately $276 billion (approximately 3.1% of the US gross domestic product). FHWA Report Number:FHWA-RD-01-156. The NACE International website has a summary slideshow of the report findings. Jones¹ writes that electrochemical corrosion causes between $8 billion and $128 billion in economic damage per year in the United States alone, degrading structures, machines, and containers.

References

• Jones, Denny (1996). Principles and Prevention of Corrosion, 2nd edition, Upper Saddle River, New Jersey: Prentice Hall. ISBN 0-13-359993-0. 
• Working Safely with Corrosive Chemicals

See also