- 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.)

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.