Histories

From- corrosion doctors
by: John Morgan, 1988

Discussion on the AC effects, Causes of AC corrosion, Prediction of AC corrosion , Mitigation of AC corrosion

Discussion on the AC effects.
The intelligent pig survey in 1999, confirmed that the corrosion was ongoing in Longton. pH measurements were carried out on the exposed pipe at the sites of the corrosion pits in Autumn of 1999. Of the ten tests carried out, three were neutral and seven were very alkaline at 11-12. This indicates the cathodic reaction 2H2O+O2+e->40H- was occurring and that the cathodic protection system was working. The high pH also indicates that SRB activity was not the cause of the corrosion as SRB's require a near neutral environment to proliferate.

This article describes the history of the investigations and how the phenomena was ultimately attributed to the effects of induced AC. Discussion and background research findings is given on probable causes of AC induced corrosion, how it can be predicted and how the effects can be mitigated against.

Alternating current induced corrosion is a significant threat to integrity of buried pipelines. The pick up of AC by buried pipelines is a well known phenomenon, in particular in areas of dense infrastructure. Inductive, capacitive, or ohmic coupling to electrical systems like power transmission lines, railways, or grounding systems of power stations may introduce high AC voltages of technical frequencies (50 or 162/3 Hz in Europe). Voltages of more than 100 VAC versus remote earth are reported, which do not only pose a hazardous situation to humans getting in direct contact, but which may also cause high AC currents across the interface metal/soil at coating defects. Such currents were found to be potentially corrosive, also for cathodically protected (CP) pipelines.

- Detecting Corrosion, Corrosion in Systems
Forty years ago, the paint companies tried using pure copper in bottom paint. It stopped marine growth cold, but it turned the entire bottom of the boat into a bonding system. Well, the EPA has us back to copper again, only this time cupric oxides are the toxic agent. Fortunately, it works pretty good. It works even better as a telltale for stray current, as the photo above shows. These oxides are still highly conductive, and still contain not completely reacted copper, so that the paints will corrode. It's true that there is no material in this world that does not corrode.

Metals are rated on which is called a Scale of Nobility. It simply means the materials ability to resist this kind of corrosion. There is also a chart called  the galvanic series, which shows the electrical potential of metals in seawater. A more noble metal is one that has a neutral or negative electrical potential. It will not generate a flow of positive ions, and is called noble. The reverse of this is the least noble metal, which has a high positive charge, and which will generate an electrical current. These include such metals as zinc, unalloyed aluminum and copper, iron and steel. Graphite and carbon bottom out the list, being the most highly charged metals.

This article is intended to give you a fundamental understanding of the causes and effects of corrosion, as well as how to identify problems and correct them before they become severely damaging. Boat owners have to deal with many types of corrosion. Actually, there are only two, but there are many different causes with different names. The two basic types are erosion and electro-chemical.

SCC is not an inevitable process, and for most metals in most environments it will not occur. Therefore it can be identified the specific combinations of metal and environment that are subject to the problem. Unfortunately, of course, as time goes by we identify more and more such combinations, especially as engineers strive to use materials more efficiently by increasing working stresses and using less expensive materials.

Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses. Three basic mechanisms of Scc have been identified.

Wood is a corrosive substance by nature and it can be made more corrosive by treatment given to it. Unlike most other corrosive substances, one of the corrosive chemicals in it, acetic acid, is volatile, and in an ill-ventilated space, wood can cause corrosion of metal nearby but not actually in contact. Where there is contact in atmospheric conditions, corrosion can occur by the usual micro-electrolytic mechanisms, and in immersed conditions, large-sized electrolytic cells can form.

It is not surprising in view of the electrochemical nature of corrosion that, measurements of the electrical properties of the metal solution interface are so extensively used across the whole spectrum of corrosion science and engineering, from fundamental studies to monitoring and control in service.

• 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.)