Table 4 Reference electrodes for use in anodic protection Electrode Solution Calomel Sulfuric acid Silver-silver chloride Sulfuric acid, Kraft solutions, fertilizer solutions Mo-MoO 3 Sodium carbonate solutions Bismuth Ammonium hydroxide solutions Type 316 stainless steel Fertilizer solutions, oleum Hg-HgSO 4 Sulfuric acid, hydroxylamine sulfate Pt-PtO Sulfuric acid Fig. 17 Potential control in the passive region for anodic protection. Source: Ref 5 Localized Attack Dissimilar-Metal Corrosion. If two metals, say aluminum and copper, are immersed in aerated chloride solutions, they will individually attain corrosion potentials of about -1.0 and -0.25 V (vs. SCE), respectively, as indicated in the galvanic series (Fig. 13). If these metals are electrically connected, then electrons will flow through the connection from the aluminum to the copper, and the aluminum will become anodically polarized and copper will be cathodically polarized with respect to their original (unconnected) corrosion potentials. The extent of this polarization and the resultant corrosion rates on the two metals will be dependent on the relative areas of the two electrodes and the conductivity of the solution. For instance, referring to the schematic Evans polarization diagrams for two metals M and N (Fig. 18), it is seen that their corrosion potentials change from the "unconnected" values E corrM and E corrN toward a common value E corr-couple , which is determined by the criterion that the total reduction current is balanced by the total oxidation current at the various parts of both surfaces. The result is that the corrosion current on the more active metal is increased from I corrM to I corrMcouple . The increase depends on the relative areas of the two metals; this important aspect can be illustrated by considering the aluminum/copper couple in aerated chloride solutions (Fig. 19). If it is assumed that the area of the two metals are equal (say 1 cm 2 ), then in the uncoupled condition the corrosion rate on the aluminum is about 0.7 A/cm 2 (0.3 mil/year), but when connected to the copper, the corrosion potential will increase from -1.05 V (vs. SCE) to approximately -0.8 V (vs. SCE), and the corrosion rate on the aluminum will increase dramatically to 70 A/cm 2 (30 mils/year). However, if the area of the copper is increased by a factor of ten, then the oxygen reduction current on the copper will increase by a factor of ten, and consequently, the current density on the original 1 cm 2 of aluminum will be increased to a catastrophic 700 A/cm 2 (300 mils/year). Fig. 18 Schematic potential/current curves for metals M and N, illustrating the fact that the corrosion current on M (i corrM ) increased to i corrMcouple when connected to metal N. This current is determined by the criterion that the total oxidation and reduction currents be equal. Fig. 19 Prediction of galvanic corrosion rates of aluminum/copper couples and effect of aluminum/copper surface area ratio. Source: Ref 5 The other important factor in this phenomenon is the ionic and electronic resistivity of the overall circuit. For instance, as the ionic resistance increases, the resultant "iR" drop in the solution will eventually cause the corrosion potentials to revert to the uncoupled values. However, it is incorrect to assume that all areas of a given metal surface are at the same corrosion potential (which is created by the local metal-environment interface) because of local polarization (e.g., areas of the aluminum adjacent to the copper) or chemistry variations (e.g., oxygen consumption in crevices and cracks). Thus, if the metals are physically in contact, the area immediately adjacent to the joint will be polarized, causing the attack at dissimilar-metal joints to be localized near the joint itself. The obvious design factors in systems involving dissimilar metals are to: (a) select metals with the least difference in "uncoupled" corrosion potentials, or (b) minimize the area of the more noble metal with respect to that of the active metal, or (c) insert an electrically insulating gasket between the two dissimilar metals (Fig. 20). Fig. 20 Flange insulated to eliminate a galvanic couple. Source: Ref 10 Intergranular Attack. Metals discussed so far in this article have been regarded as homogeneous, but in fact, most engineering alloys are multiphased or contain distributions of solid-solution elements that have different chemical activities. Such metallurgical inhomogeneities occur especially at grain boundaries, either due to grain-boundary segregation or intermetallic precipitation, and these can give rise to localized intergranular corrosion. Intergranular corrosion of austenitic Fe-Cr-Ni stainless steels offers an ideal example of the phenomenon and design methods to counteract it. In this system, chromium carbide can precipitate at the grain boundary by a classical thermally activated nucleation and growth process during heat treatment or welding. This carbide precipitation, called sensitization, occurs at about 510 to 790 °C (950 to 1450 °F) and is accompanied by chromium depletion in the adjacent metal matrix such that the chromium content can fall from 18% to less than 10% Cr in a band up to 10 m from the grain boundary. This depleted zone will have markedly different corrosion properties from the adjacent high-chromium matrix. For instance, it is seen (Fig. 21) that if the chromium content falls much below 12%, then the corrosion rate in acidic solutions rises markedly in a given oxidizing potential range, and preferential corrosion will occur. This behavior will be aggravated by the fact that this narrow depleted zone will have a lower corrosion potential than the larger area of passivated high-chromium alloy connected to it, and hence further corrosion will occur due to galvanic effects, as explained in the previous section, "Dissimilar-Metal Corrosion." Fig. 21 Effect of chromium content on anodic polarization of Fe-Ni alloys of 8.3 to 9.8% Ni in 2N H 2 SO 4 at 90 °C. Source: Ref 106 This type of attack depends on two conjoint conditions, and the problem can be resolved by attention to only one of these conditions. First, grain-boundary chromium-carbide precipitates and their associated chromium-depleted zone are necessary. These conditions can be avoided by attending to such factors as the temperature/time requirements for such "sensitization," by annealing the structure to solutionize the carbide, and by making alloy compositional changes such as low carbon content (e.g., L-grade stainless steels with <0.03% carbon) or the addition of elements such as niobium or titanium, which form more stable carbides than chromium carbide. Second, the intergranular corrosion will be observed only under oxidizing potential conditions; thus, avoidance of oxidizers such as oxygen, Fe 3+ or Cu 2+ (e.g., Eq 5a, 5b, 6, 7) may well alleviate the problem. Similar intergranular attack phenomena are seen in other passive systems, where the localized attack is associated with either active depleted zones (e.g., the copper-depleted zones in Al-Cu or Al-Zn-Mg-Cu alloys or the molybdenum- depleted zones in Ni-Cr-Mo alloys) or with active precipitates (e.g., Mg 2 Al 3 in the Al-Mg alloys or MgZn 2 in Al-Zn-Mg alloys). In all instances, however, mitigation can be obtained by attention to the metallurgical conditions that give rise to the precipitation or to the specific environments in which galvanic attack occurs. Dealloying corrosion is associated with alloys whose constituents are elements having very different electrochemical activities. For instance, zinc-copper, gray cast iron, and aluminum-tin alloys give rise to phenomena classified as "dezincification," "graphitic corrosion," and "dealuminification," respectively. In two-phase structures such as / brasses and graphitic iron, the mechanism is partially galvanic (or dissimilar-metal) attack, but it is believed that in some systems, the mechanism is enhanced by combined dissolution of both elemental components and then reprecipitation of the more noble element. The resultant damage is a friable surface with large amounts of porosity. Given these component parts to the mechanism, it is apparent that potential changes can either exacerbate or mitigate the problem. Pitting, Crevice Corrosion, and Differential Aeration. Pitting and crevice corrosion arise from the creation of a localized aggressive environment that breaks down the normally corrosion-resistant passivated surface of the metal. This localized environment normally contains halide anions (e.g., chlorides) and is generally created because of differential aeration, which creates potential drops between most of the surface and occluded regions (e.g., pits, crevices, and inclusions) that concentrate the halide at discrete locations. In pitting, this localization may begin at microscopic heterogeneities such as scratches and inclusions (e.g., sulfides). Above a given potential, negatively charged anions (e.g., Cl - ) accumulate on the metal surface and can cause breakdown of the protective oxide. The breakdown mechanism continues to be a topic of research (Ref 28). Catastrophic localized breakdown occurs at a specific corrosion potential, E pit , that is a function of the material, chloride concentration, pH, and temperature (Fig. 22). Once this breakdown occurs, pit propagation can progress rapidly, because: • The environment within the pit is deaerated (i.e., at low potential), thereby setting up a potential drop between the pit and the higher-p otential surface surrounding the pit. This potential drop concentrates the aggressive anion in the pit. • Electroneutrality considerations dictate that the increase in negatively charged non-OH - anions be counterbalanced by an increase in cations, and these usually are hydrogen ions (i.e., the pH decreases). • The combination of the two factors above leads to an increased metal dissolution rate within the pit. • The high concentration of metal cations produced in the pit leads to the precipitation of, for exam ple, metal hydroxides near the mouth of the pit, which helps ensure that the severe localized environment is contained (Fig. 23). • The precipitation of noble ions on the metal surface, or within the alloy as second phases, further accelerates the corrosion rate. Fig. 22 Schematic representation of critical pitting potential, E pit , due to anodic polarization. Source: Ref 5 Fig. 23 Schematic representation of processes occurring in an actively growing pit in iron. Source: Ref 5 A very similar sequence of events occurs in crevice corrosion, the main difference being that the initiation event is associated with the creation of a localized aggressive environment in a macroscopic (often a designed-in, geometric) crevice. As with pitting, however, the mechanism of this localization is associated with deaeration (low potential) inside the crevice, coupled with an aerated (high-potential) environment outside. Environmentally Assisted Cracking Environmentally assisted cracking describes the initiation and subcritical crack propagation in structural metals due to the combined action of tensile stress, material microstructure, and environment. The conditions of these three parameters may be specific to a given alloy/environment system, and if one of these conditions is not met, the problem does not occur. The topic is made more difficult, however, by the superposition of various mechanisms (slip dissolution, hydrogen embrittlement) and phenomena (stress corrosion, corrosion fatigue, hydrogen embrittlement). Part of this problem is addressed in the section "Life Prediction and Management" in this article. The rest of this section addresses the understanding of the mechanisms of environmentally assisted cracking and how this can be used qualitatively to control cracking. Candidate Crack-Propagation Models. The basic premise for all of the proposed crack-propagation mechanisms for ductile alloys in aqueous solutions is that the crack tip must propagate faster than the corrosion rate on the unstrained crack sides. If this were not true, the crack would degrade into a blunt notch (Ref 29, 30). Indeed, the suppression of both stress corrosion and corrosion fatigue in many systems can be explained in terms of blunting of cracks during the early propagation stage. For instance, low-alloy steels will not exhibit stress corrosion in acidic or concentrated chloride solutions unless the general corrosion/blunting effect is counteracted with chromium or nickel alloying additions (Ref 31, 32). Similar blunting explanations can be proposed for the case of corrosion-fatigue crack initiation of aluminum in chlorides in comparison to hydroxides (Ref 33). Numerous crack propagation mechanisms were proposed in the period 1965-1979 (Ref 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44). With the advent in recent years of more sensitive analytical capabilities, however, many of the earlier cracking hypotheses have been shown to be untenable. The candidate mechanisms for environmentally assisted crack propagation (for both stress corrosion and corrosion fatigue) have been narrowed down to slip dissolution, film-induced cleavage, and hydrogen embrittlement. Qualitative Prediction Methods for Environmentally Assisted Cracking in Ductile Alloy/Aqueous Environments. Qualitative predictions of cracking have centered around the observation that the rate-determining step in all of the above cracking mechanisms is not necessarily the atom-atom rupture process itself, but is one (or a combination) of mass transport of species to and from the crack tip, passivation reactions, and the dynamic strain processes at the crack tip (Ref 29, 30). Thus, changes in cracking susceptibility for most ductile alloy/aqueous environment systems with, for instance, changes in temperature, electrode potential, stressing mode, or environmental composition, can be explained logically (Ref 29, 30) using a reaction-rate surface (Fig. 24), regardless of the specific atom-atom rupture mechanism at the crack tip. This fact can be reinterpreted in terms of the crack propagation rate/stress intensity (Fig. 25) (Ref 29) relationship for a given alloy/environment system subjected to different loading histories in which the limiting and rate-controlling reactions can be defined (Ref 29). Fig. 24 Schematic reaction-rate surface illustrating the variation in crack-propagation rate with the rate- controlling parameters in the slip-dissolution, film-induced cleavage and hydrogen- embrittlement mechanisms for environmentally assisted cracking in ductile alloy/aqueous environment systems. Source: Ref 29, 30 Fig. 25 Suggested variation (Ref 29) in environmentally controlled crack- propagation rate with stress intensity for various crack-tip deformation rates COD. Note the suggested rate- controlling parameters and the fact that these relationships should be bounded by a maximum crack propagation and a minimum theoretical K Iscc or K TH . The importance of passivation kinetics on crack propagation is well recognized (Ref 29, 45, 46, 47, 48, 49). Very slow passivation rates at the crack tip will promote crack blunting due to excessive dissolution on the crack sides, whereas very fast rates will minimize the amount of crack-tip penetration per oxide-rupture event. Maximum susceptibility, with high- aspect-ratio cracks, will occur at intermediate passivation rates or in regimes of barely stable passivity (e.g., near the passivation potential). The effects of potential, anion (or cation) content, and alloying addition on cracking susceptibility can be quantitatively understood by this simple concept, regardless of whether the advancement mechanism is slip dissolution or hydrogen embrittlement. For instance, cracking susceptibility in poorly passivating systems (e.g., austenitic stainless steel in caustic at high temperature, low-carbon steel in caustic or phosphate) will be increased by actions that promote passivation. Thus in these systems, cracking susceptibility will be greatest in potential ranges adjacent to active/passive transitions on a polarization curve (Fig. 26). In contrast, systems that exhibit strongly passivating behavior (e.g., aluminum alloys, austenitic stainless steels in neutral solutions) will crack most severely under potential conditions where incipient passivity breakdown occurs due to the presence of aggressive anions (e.g., chloride) (Fig. 26b). Fig. 26 Schematic electrode-potential/current- density relationship for (a) "poorly" passivating and (b) "strongly" pas sivating systems, indicating where severe cracking susceptibility in ductile alloy/aqueous environment systems is commonly encountered The fundamental importance of passivation on the crack-propagation process in ductile alloy/aqueous environment systems also indicates an analytical method of determining the potential ranges where cracking susceptibility may be severe. An example is the direct measurement of passivation rate (Ref 50), by comparing the bare surface and fully passivated dissolution rates to determine whether a high-aspect-ratio crack is possible, or by performing potentiodynamic scans at various rates. Such rapid prediction capabilities are of use in preliminary failure analyses or risk assessments, but this usefulness should be tempered by the realization that these techniques will indicate only the possibility of severe susceptibility. As indicated in Fig. 24 and 25, the rate-determining step can change as the system parameters (e.g., corrosion potential, stressing frequency, temperature) lead to an increase in crack-propagation rate. Ultimately the rate-determining step will often be liquid diffusion, either of solvating water molecules, anions, or solvated cations to and from the crack tip. Under these conditions, the propagation rate will become independent of stress intensity (i.e., the stage II region in Fig. 25) and will exhibit a temperature dependence associated with an activation enthalpy of 4 kcals/(g mole) associated with liquid diffusion. It is important from a practical viewpoint to realize that this is a limiting condition, and that the activation enthalpy can change continuously between 4 and 30 kcals/(g mole) (symptomatic of passivation control) with corresponding changes in, for instance, loading rate (Ref 29), and temperature (Ref 29). Thus, mechanistic analyses based on a specific value of activation enthalpy must be treated with caution, unless it is determined that a limiting value is being measured. References cited in this section 5. D.A. Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice Hall, 1996 10. M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986 26. R.F. Steigerwald and N.D. Greene, J. Electrochem. Soc., Vol 109, 1962, p 1026 27. H.H. Uhlig and R.W. Rene, Corrosion and Corrosion Control, 3rd ed., John Wiley & Sons, 1985, p 217 28. Z. Szklarska-Smialawska, Pitting Corrosion of Metals, NACE, 1986 29. F.P. Ford, "Mechanisms of Environmental Cracking Peculiar to the Power Genera tion Industry," Report NP2589, EPRI, 1982 30. F.P. Ford, Stress Corrosion Cracking, Corrosion Processes, R.N. Parkins, Ed., Applied Science, 1982 31. R.N. Parkins, N.J.H. Holroyd, and R.R. Fessler, Corrosion, Vol 34, 1978, p 253 32. B. Poulson and R. Robinson, Corr. Sci., Vol 20, 1980, p 707 33. J. Congleton, "Some Aspects of Crack Initiation in Stress Corrosion and Corrosion Fatigue," paper presented at Corrosion 88, NACE, St. Louis, 21-25 March 1988 34. Conf. Proc., Environmental-Sensitive Mechanical Behavior (Baltimore, MD, June 1965), A.R.C. Westwood and N.S. Stoloff, Ed., Gordon and Breach, 1966 35. R.W. Staehle, A.J. Forty, and D. Van Rooyen, Ed., The Fundamental Aspects of Stress- Corrosion Cracking, Ohio State University, Sept 1967 36. J.C. Scully, Ed., Theory of Stress Corrosion Cracking, NATO, Brussels, March 1971 37. O. Devereaux, A.J. McEvily, and R.W. Staehle, Ed., Corrosion Fatigue Chemistry, Mechanics and Microstructure, University of Connecticut, Storrs, June 1971 38. M.P. Bastein, Ed., L'Hydrogene dans les Metaux, Science et Industrie, Paris, 1972 39. L.M. Bernstein and A.W. Thompson, Ed., Hydrogen in Metals, L, American Society for Metals, 1973 40. R.W. Staehle, J. Hochmann, R.D. McCright, and J.E. Slater, Ed., Stress-Corrosion Crac king and Hydrogen Embrittlement of Iron-Base Alloys, NACE, 1977 41. A.W. Thompson and I.M. Bernstein, Ed., Proc. Effect of Hydrogen on Behavior of Materials (Jackson Lake, WY, Sept 1975), TMS, 1976 42. R.M. Latanision and J.T. Fourie, Ed., Surface Effects on Crystal Plasticity (Hohegeiss, Germany, 1975), Noordhof-Leyden, 1977 43. P.R. Swann, F.P. Ford, and A.R.C. Westwood, Ed., Mechanisms of Environment Sensitive Cracking of Materials, The Metals Society, April 1977 44. Corrosion Fatigue, Met. Sci., Vol 13, 1979 45. T.R. Beck, Corrosion, Vol 30, 1974, p 408 46. R.W. Staehle, in Theory of Stress Corrosion Cracking, J.C. Scully, Ed., NATO, Brussels, March 1971 47. J.C. Scully, Corros. Sci., Vol 8, 1968, p 771 48. D.J. Lees, F.P. Ford, and T.P. Hoar, Met. Mater., Vol 7, 1973, p 5 49. J.R. Ambrose and J. Kruger, J. Electrochem. Soc., Vol 121, p 1974, p 599 50. F.P. Ford and M. Silverman, Corrosion, Vol 36, 1980, p 558 106. K. Osozaawa and H.J. Engell, Corros. Sci., Vol 6, 1966, p 389 Design for Corrosion Resistance F. Peter Ford and Peter L. Andresen, General Electric Corporate Research and Development Center; Peter Elliott, Corrosion and Materials Consultancy, Inc. Engineering Design Principles The earlier sections of this article provide a background about the principles of corrosion. This section focuses on engineering aspects of design that can, without due care and attention, precipitate unexpected premature failure. More extensive texts relating specifically to design are available (Ref 51, 52, 53, 54), as are guides from various material suppliers and promoters (Ref 55, 56, 57). However, it is the fine details of engineering design, often compounded by human errors or poor communication (Ref 22, 58, 59, 60, 61, 62, 63, 64), that account for many unexpected failures, at times significant. On occasion a poor design can cause premature failure of the most advanced corrosion-resistant materials. Design Considerations Designing for corrosion control can only be effective if it is part of the overall design philosophy. However, a designer is seldom a corrosion engineer, so it is necessary to convey corrosion knowledge to the designer. Unlike conventional engineering, the basic difficulty is that corrosion is not a tangible property; it is more a behavioral pattern. Thus, to realize safe, reliable designs, it is essential that there be a rigid control on materials and fabrication and an extensive effort to eliminate human errors or misunderstandings that result from poor communication. The results of a survey of chemical-process plants (Ref 65) showed that design faults ranked highest (58%) in the reasons for failure. Of almost equal ranking was the incorrect application of protective treatments (55%), followed by categories that demonstrate a lack of knowledge about the operating conditions (52%), lack of process control (35%), and an unawareness that there was actually a corrosion risk (25%). In an ideal world, designers would call for some corrosion assessment prior to preparing the detailed engineering design. Typically, schemes would permit some form of evaluation with respect to both function and the necessary action, for example from the proposal-to-production planning stages (Fig. 27) (Ref 65). In the practical "real" world, however, communication of "agreed" reasons for failures may not always reach the designer. Indeed, communication to contractors, who are closest to the application, is even poorer (Ref 65, 66). Studies have shown that, while management is always informed of the reasons for failure in the chemical-processing industry, site personnel are informed only 77% of the time, designers 55%, material suppliers 37%, and contractors only 11% of the time. [...]... Science, 1982 51 V.R Pludek, Design and Corrosion Control, MacMillan, 1977 52 R.J Landrum, Fundamentals of Designing for Corrosion Control, NACE International, 1989 53 R.N Parkins and K.A Chandler, Corrosion Control in Engineering Design, Department of Industry, Her Majesty's Stationery Office, London, 1978 54 L.D Perrigo and G.A Jensen, Fundamentals of Corrosion Control Design, The Northern Engineer,... teams having detailed procedures and including qualified surveyors, inspectors, and supervisors Ensure that standby products are available, fully labeled, and properly stored (using dessicants and noncorrosive packaging) As noted above, the planned approach for reliable engineering design should include corrosion control and/ or preventive measures, for which standards and specifications are available... p 2 Design for Corrosion Resistance F Peter Ford and Peter L Andresen, General Electric Corporate Research and Development Center; Peter Elliott, Corrosion and Materials Consultancy, Inc Life Prediction and Management As pointed out in previous sections, corrosion degradation is largely understood mechanistically, and logical mitigation actions and design decisions can be formulated with a reasonable... considering materials, it is important to avoid nonspecific descriptions or terms in reference to design drawings and specifications There are many instances where generic terms, such as "stainless steel," "bronze," "Hastelloy," or "Inconel," are too vague and the ultimate choice is far from what was expected and required Wherever possible, and notably in high-risk areas, materials should be selected and tested... Buried structures can be affected where soil and bacterial corrosion might apply (Ref 72) Design and Materials Selection Corrosion control measures are best initiated at the design stage (Fig 27) Materials are usually selected to perform a basic function or to provide a functional requirement (see the article "Materials Selection" in Ref 22) Therefore, in many instances the material choice is dictated not... (Table 6) Material selection for high-temperature service needs to be reviewed for each individual part and application Alloy steels and more sophisticated alloys based on nickel and cobalt are most commonly used, in which key elements for high-temperature corrosion resistance include chromium, aluminum, silicon, and rare-earth additions for scale retention Table 6 Types of corrosion and corrodents encountered... sophisticated products that require careful mixing and application Maintenance procedures frequently require field application where some control (use of trained inspectors) is essential, as in offshore oil and gas rigs Inspection codes and procedures are available and total design should incorporate these wherever possible In critical areas, design for online monitoring and inspection will also be important The... be willing to take action and the designer should insist on appropriate codes and/ or recommended working practices Whether the necessary action will be taken is affected by financial, technical, safety, social, and/ or political issues (Ref 69) To prevent corrosion/degradation, the designer can: • • • • • Avoid obvious design weaknesses (see examples below) Use more reliable materials, even if this entails... Engineer, Vol 13 (No 4), 1982, p 16 55 Designer Handbooks, Specialty Steel Industry of North America, Washington, D.C.; also publications relative to design, Nickel Development Institute, Toronto, Canada 56 Guides to Practice in Corrosion Control, Department of Industry, Her Majesty's Stationery Office, London, 1979-1986 57 Engineering Design Guides, Design Council, British Standards Institute, Council of Engineering... Understanding Corrosion Attack, Plant Eng., Oct 1993, p 68 65 P Elliott, Corrosion Survey, Supplement to Chem Eng., Sept 1973 66 P Elliott, Catch 22 and the UCS Factor Why Must History Repeat Itself?, Mater Perform., Vol 28 (No 7), 1989, p 70 and Vol 28 (No 8), 1989, p 75 67 Standards for Corrosion Testing of Metals, ASTM, 1990 68 R Baboian, Ed., Corrosion Tests and Standards: Applications and Interpretation, . bacterial corrosion might apply (Ref 72). Design and Materials Selection Corrosion control measures are best initiated at the design stage (Fig. 27). Materials are usually selected to perform. occasion a poor design can cause premature failure of the most advanced corrosion-resistant materials. Design Considerations Designing for corrosion control can only be effective if it is part of. realize safe, reliable designs, it is essential that there be a rigid control on materials and fabrication and an extensive effort to eliminate human errors or misunderstandings that result from