Corrosion long-line

Reaction of iron containing a discontinuous magnetite scale with oxygenated water, crevice corrosion, water-line attack, long-line corrosion of buried iron pipes, etc. — separable A/C type. 

Although iron pipes suffer from the same corrosion risk as steel pipelines, associated with the generation of a galvanic cell with a small anode and a large cathode, the risk is mitigated for iron pipelines because the electrical continuity is broken at every pipe joint. For this reason long-line currents are uncommon in iron lines and cathodic protection is rarely necessary. It also accounts for the ability to protect iron lines by the application of nonadherent polyethylene sleeving . 

On bare or ineffectively coated pipelines where long-line corrosion activity is of primary concern, the measurement of a net protective current at predetermined current discharge points from the electrolyte to the pipe surface, as measured by an earth current technique, may be sufficient. 

Along the length of the pipe, variations in soil resistivity may stimulate localized metal corrosion and can cause long line currents in the area of lower resistivity soil the buried metal surface is usually anodic, with a more negative potential, compared to that in the soil of higher resistivity. 

The difference in activity coefficient between the aqueous and the non-aqueous media is enough to confer some potential on the electrodes and cause corrosion of one of them. This sort of effect is observed in metal pipes that run a considerable distance and pass from one kind of soil into another. The resulting corrosion is then due to what is called long line currents . When it is possible to put the pipes into the same type of soil all the way, e.g. by filling the trenches with sand or gravel before laying the pipes, this corrosion is prevented. 

Projection radiography has long been used for pipe inspection and corrosion monitoring. In this traditional tangential wall thickness estimation the distance of border lines of the projected wall shadows of a pipe onto the film is a direct measure for the wall thickness. This method is not considered here, newer developments can be found in / /. 

The principles pertaining to carbon blast furnace hearths apply as well to submerged-arc furnace hearths. In some processes, such as in d-c arc furnaces, the electrical conductance of carbon is a most important factor. The long life of carbon linings in these appHcations is attributable to carbon s exceptional resistance to corrosive slags and metals at very high temperatures. 

Cost rules out almost all alternative materials for long-distance pipe lines it is much cheaper to build and protect a mild steel pipe than to use stainless steel instead - even though no protection is then needed. The only competing material is a polymer, which is completely immune to wet corrosion of this kind. City gas mains are now being replaced by polymeric ones but for large diameter transmission lines, the mechanical strength of steel makes it the preferred choice. 

The choice of material from the viewpoint of mechanical properties must be based on design conditions. However, from a corrosion standpoint it must be realised that the design conditions are limiting values and that for most of its life the equipment will operate under process conditions . For the decision on the requisite corrosion-resistance properties it is necessary to examine, by means of an operability study, how far process conditions may deviate from the normal and how often and for how long. The operability study is carried out using a line diagram for the projected plant. 

Materials may be subject to intense localised attack at the liquid level when they are partially immersed in a solution under conditions where the water line remains at a fixed position for long periods. This attack may be the result of concentration cell effects complicated by differences in the nature and adherence of corrosion-product films as they form in the water-line region as compared with those that form above or below this region. 

The RP tank just reviewed that was removed in 1988 met these requirements, but two steel tanks unearthed from the same site at that time failed to meet them. There was no record of how long the steel tanks had been in service, but one was dusted with white metal oxide and the other showed signs of corrosion at the weld line. Rust had weakened this joint so much that it could be scraped away with a pocketknife. 

A long-chain amine usually fed to the steam header and employed to prevent steam and condensate line corrosion by a surface-filming... 

The sample loop, Vg, has a volume of 0.25 cc. To initiate injection at time zero of the experiment, the valve handle is rotated 60°, and the flows within the valve are redirected along the dashed lines as shown in the figure. The sample solution in the loop is then displaced through port F by the 1% acidified brine from the pump. The sample loop therefore must be long enough to contain a sufficient "slug" of sample solution. Here, the phrase "sample solution" refers to the surfactant solution prepared in 1% acidified brine solution. Teflon tubing and connections are used to minimize the uptake of any corrosion products into the 1% acidified brine solution. 

Transfer optics The interface of the IR beam with the sample, especially in a classified hazardous environment, can be a major challenge. Gas samples are not too difficult although it is important to pay attention to the corrosivity of the gases, relative to the windows and any internal optics, as with a folded path length cell. Liquids offer different challenges. For on-line applications users prefer to minimize the use of valves, bypass streams and auxiliary pumps, especially over long distances between the stream and the analyzer. At times there is a benefit to sample the stream either directly or as close as possible... 

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