7.6 Corrosion of metals and alloys

The electrochemical series ranks the general resistance of metals to corrosion. The more negative the standard Emf (Electromotive force) potential, the more easily the material will oxidize. The monocrystal structure shows a higher resistance to corrosion than the same metal with polycrystalline structure. The smaller the size of grains, the more the material is prone to corrosion damages. If two metals are electrically connected and immersed in a solution of their own ions the EMF potential determines which material will corrode. Iron dissolves in the electrolyte because iron has electrode potential (-0.44 V) lower than that of copper (0.33 V). Consquently, the copper deposits on the cathode. The magnitude of the voltage driving the dissolution of iron is found to be:

DV = V₁ - V₂ = 0.34 - (-0.44) = 0.78 V

Galvanic corrosion occurs when dissimilar metals are placed in assembly within a corrosive electrolyte (e.g. sea water). This results in one of the metals becoming anodic and corroding at faster rate than normal. The other metal is the cathode responds with a decrease in corrosion rate. The Galvanic series is useful for selecting materials to be joined. Materials towards the bottom of the table are more active (anodic) and will corrode at a faster rate than those above them. In addition, the closer two metals are in the table the weaker the corroding effect. Corrosion rate depends on the relative areas of the anode and cathode. When the surface area of the anodic metal is smaller than that of the cathode the resulting corrosion is rapid. Consequently, the corrosion rate is slow when a larger anode is connected to a small cathode. The bolt displayed under a constant load will corrode at a greater rate than one that is unloaded. This is due to regions of a high local stress being anodic to those of a lower stress. The combined action of a sufficient applied tensile stress and an aggressive environment can cause the cracking of a part. Areas of metals subjected to cold working are rich in dislocations and therefore constantly under stress. This results in them being anodic to the less stressed regions and accelerates corrosion. The flow of oxygen to the area under the gasket is restricted and therefore its concentration is low. This area will be anodic and corrode faster than the oxygen rich areas. For a given fatigue life the influence of a corrosive environment on the fatigue strength of metals increases if the frequency decreases. This means that a structure loaded at a lower frequency will sustain less cycles to fracture at a given applied stress. The picture shows S-N curves of carbon steel tested in several mediums. All metals and alloys cyclically loaded under a corrosive environment do not exhibit a endurance (fatigue) limit. Which means that a structure exploited under such conditions will finally break even if applied stress is very low.

7.7 Casting

The typical structure of a cast alloy consists of three zones: 1. Chill zone - a few layers of fine equiaxed grains near the mold walls. 2. Columnar zone - oriented grains grown in the direction opposite of the heat transfer through the mold. 3. Equiaxed zone - equiaxed grains of large size at the center of the casting. Depending on the processing conditions and material the proportion of the columnar and equiaxed zones can be altered. Slow cooling, adding the nucleating agents and agitating the melt contribute to the growth of equiaxed zone. The enlarged columnar zone is peculiar for pure metals. The greater the volume to surface area ratio, the slower a solid body cools and solidifies. Solidification time can be estimated by Chvorinov's rule:

T_S = B(V/A)²,

where V is the volume; A is the surface area; B is an empirical constant. Patterns very often have a temper on the vertical surfaces parallel to the direction of withdrawal. This allows for an easy removal of the pattern from the mold without any distortion or breaking of the mold cavity. The angle of draft is normally 0.5-2^o. The angle depends mainly on the materials and processing conditions. Materials with a short temperature range of crystallization (eg. pure metals or eutectic alloys) tend to form a large concentrated shrinkage cavity (right). The castings of alloys with a large freezing range have porosity dispersed in the bulk of the material (left). The fluidity is the capability of material to flow into mold cavities prior to solidification. The fluidity of pure metals and eutectic alloys is higher than that of hypoeutectoid or hypereutectoid alloys. Risers are used to compensate for the shrinkage of molten metal during solidification and to avoid the formation of a shrinkage cavity within the casting. The shrinkage cavity forms into the riser because it is the last part solidified in the mold. The risers are usually located over the center of the heaviest sections of castings. The riser must be large enough to feed the shrinkage in the casting. Solidification shrinkage varies for different metals and influences the size of the risers.

Permanent mold casting vs Sand casting:

Increased dimensional accuracy and smoother surfaces; A new mold to produce every part is avoided; Increased mechanical properties due to a fine grain structure; Less time to cast a part; Shape and size of castings are limited; Not suitable for metals with a low fluidity.