There are media in which the passive layer might break down locally, while the rest of the layer remains intact. Such media cause so-called localized corrosion. Pitting corrosion is one type of localized corrosion (later we describe more types) and is characterized by attack at small discrete areas. Attack of this kind occurs mainly in neutral or acidic solutions containing chloride.
Chloride ions facilitate a breakdown of the passive layer. A break in the passive film may be considered as a galvanic cell, in which the bare metal becomes the anode while the surrounding area, with an undamaged passive layer, becomes the cathode. This unfavourable anode-to-cathode surface area ratio causes rapid corrosion of the anode.
When the metal corrodes, dissolved metal ions generate an environment with a low pH and chloride ions migrate into the pit to balance the positive charge of the metal ions. Thus the environment inside a growing pit gradually becomes more aggressive and repassivation becomes less likely. As a result, pitting attack often propagates at a high rate, thereby causing corrosion failure in a short time. Since the attack is small at the surface and may be covered by corrosion products, a pit often remains undiscovered until it causes perforation and leakage.
4. Pitting attack in the heat affected zone inside a but-welded stainless steel pipe.
Crevice corrosion occurs under the same conditions as pitting, i.e. in neutral and acid chloride solutions. However, attack starts more easily in a narrow crevice than on an unshielded surface. The chemical reactions occurring naturally on a stainless surface in an aqueous environment consume oxygen. In the stagnant solution inside a crevice, the supply of new oxidant is restricted. The composition of the solution within the crevice might thus gradually become different from that of the ambient solution. This difference in composition increases the risk for corrosion. Small amounts of dissolved metal ions cause a decrease of the solution pH inside the crevice and the presence of chlorides facilitates an activation of the metal surface. The bare metal surface behaves anodically to the passive areas and the attack propagates according to the same mechanisms as in the case of pitting. Crevice corrosion may be strong even at relatively low temperature.
High chloride concentrations and low pH increase the probability of pitting and crevice corrosion, as do high temperatures and stagnant solutions. The resistance of stainless steels to these types of corrosion increases with increasing contents of chromium and molybdenum. For austenitic and duplex grades, high nitrogen content is also very beneficial. We draw the attention to the fact that a smooth and clean surface generally exhibits the best corrosion resistance. Since crevices are the most vulnerable areas to corrosion, it may be advantageous to use more highly alloyed materials (i.e. 316 instead of 304) in joints (like flanges, bolts etc) than in the pipes of a piping system. The resistance of a construction to crevice corrosion may also be enhanced by overlay welding with a more corrosion resistant material at potential sites of crevice corrosion. Finally, we note that pitting attack initiates more easily in stagnant solutions than in flowing media and that is why equipment should be designed in a way that permits complete drainage.
For the right examination and decision of which type to choose for avoiding the most common form of corrosion - which is pitting - we use the so-called PRE factor. PRE (Pitting Resistance Equivalent) constitutes a measurement of a stainless steel grade's relative resistance to pitting corrosion. The highest is a grade's PRE number, the highest corrosion resistance it enjoys. The formula that gives us the PRE of each grade is the following
PRE = % Cr + 3.3 % Mo + 16 % N
and shows that the resistance to corrosion is the result of the steel's chemical composition and not its atomic structure (austenitic or ferritic). Notable is the absence of nickel in the PRE formula, given that in most cases it does not contribute to the evasion of pitting corrosion.
5. PRE comparison between popular and austenitic grades.
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