Service Environment

Nondestmctive tests differ from methods of laboratory analysis and testing where specimens are generally sectioned, broken, damaged, or destroyed. Nondestmctive tests can be performed on materials, components, and stmctures or systems that actually are to be used. Thus, effective use of NDE requires engineering knowledge of the stmcture, the performance characteristics, and service environment, as well as the test method. More complete information on all of the topics discussed herein are available (1 6). 

One of the principal reasons for failure due to reaction with the service environment is the relatively complex nature of the reactions involved. Y"et, in spite of all the complex corrosion jargon, whether a metal corrodes depends on the simple elec trochemical cell set up by the environment. This might give the erroneous impression that it is possible to calculate such things as the corrosion rate of a car fender in the spring mush of salted city streets. Dr. M. Pourbaix has done some excellent work in the application of thermodynamics to corrosion, but this cannot yet be applied direc tly to the average complex situation. 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

Many shell-and-tube condensers use copper alloy tubes, such as admiralty brasses (those containing small concentrations of arsenic, phosphorus, or antimony are called inhibited grades), aluminum brasses, and cupronickel austenitic stainless steel and titanium are also often used. Utility surface condensers have used and continue to use these alloys routinely. Titanium is gaining wider acceptance for use in sea water and severe service environments but often is rejected based on perceived economic disadvantages. 

Substituting one alloy for another may be the only viable solution to a specific corrosion problem. However, caution should be exercised this is especially true in a cooling water environment containing deposits. Concentration cell corrosion is insidious. Corrosion-resistant materials in oxidizing environments such as stainless steels can be severely pitted when surfaces are shielded by deposits. Each deposit is unique, and nature can be perverse. Thus, replacement materials ideally should be tested in the specific service environment before substitution is accepted. 

Although resistance to deflection and plastic yielding are obviously of first importance in choosing alternative materials, other properties enter into the selection. Let us look at these briefly. Table 27.4 lists the conditions imposed by the service environment. 

Uniform corrosion is the deterioration of a metal surface that occurs uniformly across the material. It occurs primarily when the surface is in contact with an aqueous environment, which results in a chemical reaction between the metal and the service environment. Since this form of corrosion results in a relatively uniform degradation of apparatus material, it can be accounted for most readily at the time the equipment is designed, either by proper material selection, special coatings or linings, or increased wall thicknesses. 

Materials evaluation should be based only on actual data obtained at conditions as close as possible to intended operating environments. Prediction of a material s performance is most accurate when standard corrosion testing is done in the actual service environment. Often it is extremely difficult in laboratory testing to expose a material to all of the impurities that the apparatus actually will contact. In addition, not all operating characteristics are readily simulated in laboratory testing. Nevertheless, there are standard laboratory practices that enable engineering estimates of the corrosion resistance of materials to be evaluated. 

Environmental composition is one of the most critical factors to consider. It is necessary to simulate as closely as possible all constituents of the service environment in their proper concentrations. Sufficient amounts of corrosive media, as well as contact time, must be provided for test samples to obtain information representative of material properties degradation. If an insufficient volume of corrosive media is exposed to the construction material, corrosion will subside prematurely. 

For galvanic corrosion tests it is important to maintain the same ratio of anode to cathode in the test sample as in the service environment. 

Select materials based on their functional suitability to the service environment. Materials selected must be capable of maintaining their function safely and for the expected life of the equipment, and at reasonable cost. 

Thorough assessment of the service environment and a review of options for corrosion control must be made. In severe, humid environments it is sometimes more economical to use a relatively cheap structural material and apply additional protection, rather than use costly corrosion-resistant ones. In relatively dry environments many materials can be used without special protection, even when pollutants are present. 

For service environments in which erosion is anticipated, the wall thickness of the apparatus should be increased. This thickness allowance should secure that various types of corrosion or erosion do not reduce the apparatus wall thickness below that required for mechanical stability of the operation. Where thickness allowance cannot be provided, a proportionally more resistant material should be selected. 

The first and most important steps in the design process are to define clearly the purpose and function of the proposed product and to identify the service environment. Then one has to assess the suitability of a range of candidate materials. The following are generally regarded as the most important characteristics requiring consideration for most engineering components. 

A wide variety of thermoplastics have been used as the base for reinforced plastics. These include polypropylene, nylon, styrene-based materials, thermoplastic polyesters, acetal, polycarbonate, polysulphone, etc. The choice of a reinforced thermoplastic depends on a wide range of factors which includes the nature of the application, the service environment and costs. In many cases conventional thermoplastic processing techniques can be used to produce moulded articles (see Chapter 4). Some typical properties of fibre reinforced nylon are given in Table 3.2. 

In a service environment if there is any equipment upon which the capability of your service depends, this equipment should be maintained. Maintenance may often be subcontracted to specialists but nevertheless needs to be under your control. If you are able to maintain process capability by bringing in spare equipment or using other available equipment, your maintenance procedures can be simple. You merely need to ensure you have an operational spare at all times. Where this is not possible you can still rely on the call-out service if you can be assured that the anticipated downtime will not reduce your capability below that which you have been contracted to maintain. 

Select compatible materials for the service environment and conditions. 

One of the most effective methods of preventing corrosion is the selection of the proper metal or alloy for a particular corrosive service. Once the conditions of service and environment have been determined that the equipment must withstand, there are several materials available commercially that can be selected to perform an effective service in a compatible environment. Some of the major problems arise from popular misconceptions for example, the use of stainless steel. Stainless steel is not stainless and is not the most corrosion-resistant material. Compatibility of material with service environment is therefore essential. For example, in a hydrogen sulfide environment, high-strength alloys (i.e., yield strength above 90,000 psi or Rc 20 to 22) should be avoided. In material selection some factors that are important to consider are material s physical and chemical properties, economics and availability. 

The first source of information for the behavior of a material in the proposed service environment is the potential supplier of the item of plant. Except for new (or significantly modified) processes, specialist suppliers or fabricators have relevant information and service experience. The supplier should be provided with all process or environmental details that are of possible relevant to corrosion. The most important are listed below ... 

Stainless steels are used in a wide variety of applications and are most often selected because steel or cast iron would corrode at an unacceptably high rate or produce high levels of iron contamination in the proposed service environment. The main limitations on their uses are ... 

Vast amounts of continuously galvanised steel sheets are produced, and unless they are painted or otherwise coated, their life depends on the thickness of the galvanising and the service environment in which they are used. Similarly in the case of steel sheets coated with aluminium or aluminium-zinc alloys, their performance is dictated by their coating thickness (see Section 13.4). A problem often associated with such material is corrosion at the cut edges. From work carried out by BISRA and others it has been shown that providing the bare steel edge is less than 3 mm in width, the amount of corrosion is minimal and the life of the sheet is not adversely... 

The use of electrodeposited metals to protect corrodible basis metals from their service environments has been well established for many years and accounts for by far the larger part of the activities of the plating industry. There are many reasons for using an electroplated metal finish in preference to an organic finish or to making the articles concerned from inherently corrosion-resistant materials. 

Physical properties The corrosion resistance of an electrodeposit depends not only on suitability for the service environment, thickness, and uniformity, but also on its physical and mechanical qualities such as adhesion to... 

Because of the many variables which can influence the corrosion reaction, the use of the e.m.f. series of metals to predict the behaviour of galvanic couples in a given service environment can be hazardous and misleading. Numerous examples of coatings expected to act cathodically which have, in fact, been anodic have been reported in the literature and specialised lists of galvanic couples in different environments have been compiled. ... 

Up to 750°C, the performance of all aluminium diffusion coatings is considered to be very good, but above this temperature the results appear to be dependent on the coating thickness, diffusion treatment and the specific service environment. Sprayed aluminium coatings can be used up to 900°C after diffusion treatment. Hot-dipped coatings also benefit from additional diffusion treatment, and omission of silicon from the coating alloy improves performance at the elevated temperatures. 

With some materials, there are specific heat treatments that are known to reproduce the worst effects of the heat of welding. It is recommended, therefore, that in tests made to qualify a material for a particular service environment, in addition to the exposure of welded test specimens in order to observe effects of welding heat, specimens should be included that have been given a controlled abusive or sensitising heat treatment. As an illustration, austenitic stainless steels may be held at 650-700° forO-5-1 h, followed by testing for susceptibility to intercystalline attack as in ISO 3651-1 or -2 1976. 

Streicher , however, considered this approach to be unsound and pointed out that the short duration of the potentiostatic studies carried out by France and Greene cannot be used to predict long-term behaviour in service. The prolonged dialogue between these workers was well summarised in the review article by Cowan and Tedmon who concluded that these particular potentiostatic tests cannot be regarded as accelerated tests for service environments and that predicting future industrial service for periods longer than the test is not advisable. 

Identity specific functions Dimensions, structural loads, govl/industry standards, service environments, etc. 

Gradient across component thickness No. of cycles—high to low No. of cycles—freeze-thaw Solar gain, surface air flow Liquid, moisture, and/or vapor tightness Strength-weight ratios—relative significance Service environment ... 

Establish final design criteria Allowable stresses, strains, deflections Margins of safety against local and overall instability, vibrations, etc. Take into account type and duration of load, service environments, process effects, equality expectations... 

Test materials for structural properties and effect of service environment... 

Determine that materials produced in actual fabrication process will have the minimum structural properties and resistance to service environment assumed in the design. Extent of testing, if any, depends on available information about specific materials and processes to be used... 

Protect or modify materials if service environment causes excessive degradation of properties... 

---