Corrosion creep results

The effect of interface of steel on the corrosion test result is summarized in Table 33.2. The results found for the panels, which were E-coated without phosphate or plasma polymer coatings, are quite astonishing. A scribe creep of 3.0 mm in a 4-week corrosion test was obtained for an oxide-removed CRS surface (without zinc phosphate nor plasma polymer), which is better than the E-coat on the... 

Table 33.2 Effect of the Presence or Absence of Oxides on the Corrosion Test Result Scribe Creep Width on GM Scab Test...

Improving high temperature corrosion resistance Table 11.2 Creep results for P92 at 650°C/120 MPa... 

Thermal fatigue characteristically results from temperature cycles in service. Even if an alloy is con ectly selected and operated within normal design limits for creep strength and hot-gas corrosion resistance, it can fail from thermal fatigue. 

Rust Creep-also called underfilm corrosion refers to corrosive action that results in damaged or uncoated areas and extends subsequently under the surrounding inert protective coating. 

The protection of vessels containing corrosive materials presents a special problem for the selection of bursting discs—a rapid rate of corrosion can lead to a high frequency of failures. In addition, the creep of a metal disc when under tension at elevated temperatures would tend to weaken it and result in premature failure. 

The implications of a significant role for strain rate are wider than the obvious one that stress corrosion should only occur over a restricted range of strain rates. Thus, in constant load tests, since cracks will continue to propagate only if their rate of advancement is sufficient to maintain the crack-tip strain rate above the minimum rate for cracking, it is to be expected that cracks will sometimes stop propagating, particularly below the threshold stress. Such non-propagating cracks are indeed observed below the thres-hold . Moreover, in constant-load or constant-strain tests, the strain rate diminishes with time after loading, by creep exhaustion if the stress remains sensibly constant, and it is found that the stress-corrosion results are sensitive to the relative times at which the stress and electrochemical... 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Dressing Sharp edges must be removed. Thermoplastics have a greater coefficient of thermal expansion than metals. They therefore shrink onto the metal and if sharp edges are present then these will cut through the coating and become exposed. These exposed edges will start to corrode and this will inevitably result in underfilm creep corrosion. 

For the cyclic corrosion test, a layer of acrylosilane polymer coating (10-25 fim thick) was dip-coated onto the plasma-deposited substrates. The coated samples were then subjected to 25 scab cycles. The test results are plotted in Fig. 7. Corrosion performance (as described by the length of scribe creep) was correlated to the wattage used for plasma film deposition. As discussed in the previous section, the chemical structure and properties correlated with the deposition conditions, especially the power level applied. Therefore, atomic compositions for plasma polymers deposited at different power levels were also plotted in Fig. 7.A... 

Pure lead has low creep and fatigue resistance, but its physical properties can be improved by the addition of small amounts of silver, copper, antimony, or tellurium. Lead-clad equipment is in common use in many chemical plants. The excellent corrosion-resistance properties of lead are caused by the formation of protective surface coatings. If the coating is one of the highly insoluble lead salts, such as sulfate, carbonate, or phosphate, good corrosion resistance is obtained. Little protection is offered, however, if the coating is a soluble salt, such as nitrate, acetate, or chloride. As a result, lead shows good resistance to sufuric acid and phosphoric acid, but it is susceptible to attack by acetic acid and nitric acid. 

What is the use of an optimum surface treatment if creep corrosion affects the adhesive layer from outside the glueline, which results in the destruction of the bonded joint This process is shown in Figure 7.8. 

In selecting metals and alloys as materials of construction, one must have knowledge of how materials fail, for example is, how they corrode, become brittle with low-temperature operation, or degrade as a result of operating at high temperatures. Corrosion, embrittlement, and other degradation mechanisms such as creep will be described in terms of their threshold values. Transient or upset operating conditions are common causes of failure. Examples include start-ups and shutdowns, loss of coolant, the formation of dew point water, and hot spots due to the formation of scale deposits on heat transfer surfaces. Identification and documentation of all anticipated upset and transient conditions are required. 

As the grain boundaries move through the casting, they are trapped on defects and impurities. This can result in boundaries that are more prone to corrosion. It has been suggested [25] that heat treatment of the alloy can redissolve the PbsCa particles near the boundary and, thereby, reduce the rate of corrosion of the alloy. Certainly, the mechanical properties, creep rate, and corrosion rates of lead-calcium alloys are... 

---