• elevated concentrations of sulfate, chloride, ammonia compounds, or sulfide;
• poor aeration, which supports anaerobic bacteria activity;
• large amounts of organic or inorganic acid; and
• large oxygen or neutral-salt (especially chloride) differentials.

Over 98% of pipelines are buried. No matter how well these pipelines are designed, constructed and protected, once in place they are subjected to environmental abuse, external damage, coating disbondments, inherent mill defects, soil movements or instability and third party damage. In pipelines this occurs due to a combination of appropriate environment, stresses (absolute hoop and/or tensile, fluctuating stress) and material (steel type, amount of inclusions, surface roughness.)

Both methods of combating corrosion, cathodic protection (CP) and chemical inhibitors (CIs), depend on controlling the charge on the metal surface, and this can be monitored by measuring the potential of the metal. The conditions needed to stop corrosion can then be predicted from an electrochemical phase diagram.

Water Erosion
Water is the most important erosional agent and erodes most commonly as running water in streams. However, water in all its forms is erosional. Raindrops, especially in dry environments, create splash erosion that moves tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny rivulets and streams and creates sheet erosion.

Offshore oil comes from the ground in flow lines at high temperature, but then is rapidly cooled by deep water at low temperatures once it is in the subsea pipeline. For this precipitation of water  can be caused, which increases corrosivity and deposition of waxy substances also can be caused, both of which can jeopardize flow, system integrity and ongoing operations.

Constant extension rate test experiments have been performed on AISI 304 at 200°C in a 0.001 M NaCI solution. During straining, the specimens were kept at constant potentials in the range of -400 to +425 normal hydrogen electrode. Chloride SCC was seen only at potentials above +150 mV NHE.

At lower potentials, only small brittle surface cracks were formed, because slow straining at high stress levels strengthens the steel. From corrosion potential measurements at 200°C in oxygen-containing water, it follows that the critical potential value of + 150 mV NHE can be reached with oxygen contents above 10 ppb in nearly stagnant water.

Chloride Induced Stress Corrosion Cracking (SCC) of Stainless Steel Water Still

A water still failed due to water leaks in the shell, which leaks were below the water line in an area heated by an internal steam coil. A weld repair was attempted. After welding, several other cracks in the same general area opened up.

Some alloys and some specific elments are corrosion resistant. increasing the amounts of these elements, the alloys may become more corrosion resistant. Some alloys which are corerosion resistant are described as below.

For protection against microbiologically influenced corrosion, it is essential to achieve a continuous protective coating barrier that will resist highly aggressive chemicals such as sulfuric acid. Also important are adhesion, abrasion resistance, cathodic disbondment resistance, impact resistance, and low permeability.

Nitric acid is a vary strong acid and oxidant. It is particularly aggressive to most metals, and even more so with copper.

About 200 feet of 2 inch, schedule 10, Type 304 stainless steel transfer piping was installed to replace a bulk handling operation. The product, which could produce nitrous oxide (HNO2) as a breakdown product, was one that had been routinely handled at the plant for many years. Within two weeks of when the pipeline began carrying the product, the line was observed to be leaking. The insulation was stripped from 20 feet of the horizontal run on either side of the suspected leaking area and the pipe was visually inspected.  When no readily apparent leak sources could be identified, the welds attaching the 90º elbows to the straight lengths were dye penetrant inspected. The dye penetrant inspection found two pinhole sized indications in two circumferential welds.

Sulfate reducing bacteria from the genera desulfovibrio are a typical example of anaerobic MIC. In municipal wastewater systems, the SRBs, responsible for anaerobic MIC, are primarily Thiobacilli, which thrive on the sewage environment. System structures are rarely completely filled with sewage; thus, ample space exists above the water line for bacterial growth and gaseous products from the decomposition of sewage to collect. Due to their extremely rapid rate of reproduction, enormous colonies of bacteria are produced in a short time, resulting in a large potential source of corrosive media.

When you work diligently to protect your building from fire, you install a quality sprinkler system with state-of-the art piping and heads; and conduct regular inspections. Then a fire breaks out and your system fails. The tiny, but very aggressive cause may be hidden inside these pipes, bacterial microorganisms that cause corrosion known as Microbiologically Influenced Corrosion (MIC).

FHWA researchers evaluate the accuracy and reliability of three chloride test kits to determine their performance and accuracy. Following are detailed descriptions of how the researchers conducted tests using the three kits.

• Swab Test
• Patch Test
• Sleeve Test