Although atmospheres can be classified into four basic types, most of them are mixed and present no clear lines of demarcation. Furthermore, the type of atmosphere may vary with the wind pattern, particularly where corrosive pollutants are concerned.
Two of the most fundamental and informative field measurements are soil resistivity surveys and pipe-to-soil potential surveys.
In water, the concentrations of various substances in dissolved, colloidal, or suspended form are typically low, but may vary considerably depending on the components and usage. For example, hardness values of up to 400 parts per million of calcium carbonate is sometimes tolerated in public supplies of potable water, whereas 1 ppm of dissolved iron would be unacceptable.
Steel structures such as bulkheads, piles, offshore drilling platforms, etc., may be protected with either sacrificial galvanic anode or impressed current cathodic protection systems.
Several elements of preplanning are necessary to effectively execute the condition survey. The first is a logical breakdown of the facility, and the second is to have a grading system to evaluate the observed conditions.
Corrosion failures are often subtle and a result of invisible localized effects in the form of pits, intergranular corrosion, or attack within crevices.
In every industrial environment, the safety factor is an important consideration. In some environments, including the application of protective coatings, it is of overriding importance. This article explores many of the safety factors that need to be considered.
It is important to maintain inhibitor concentrations at a safe level in waters containing dissolved salts, particularly if these include chlorides.
Although the primary focus of a corrosion engineer is on the chemical stability and corrosion resistance of these materials, it is critical to cooperate with other design team members familiar with the mechanical, physical, and other properties to ensure that the desired materials performance can be achieved.
Experience has shown that even when impossible circumstances appear to be present, substantial cost savings are possible by intelligent use of corrosion data and good design practices.
The greatest use of high-temperature water and steam is in electrical power generation. Historically, fossil fuels were used almost exclusively to heat water and make steam until the introduction of nuclear power steam generators in the second part of the 20th century.
Corrosion Basics don’t get any more basic than the definition of corrosion as the deterioration of a material, usually a metal, that results from a chemical or electrochemical reaction with its environment. Almost all materials should be expected to deteriorate, to some extent, with time when exposed to the elements. Corrosion is a perfectly natural process, as natural as water flowing downhill.
Although the corrosion rate of bare steel tends to decrease with time in most cases, the difference in corrosivity of different atmospheres for a particular alloy is tremendous.
Steel is fairly uniform in texture, and workers are familiar with the proper ways to prepare and coat it. In contrast, concrete is quite porous and uneven in texture, with differences in density from one square inch to the next.
General, or uniform, corrosion is usually the least threatening type of attack because associated metal loss is distributed over a wide surface area. Unfortunately, most corrosion encountered in engineering practice is of a more localized type. Localized corrosion can be defined as selective attack at limited special areas or zones on a metal surface in contact with an environment.