Pitting corrosion formation

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

Rozenfd d, I. L. and Danilov, I. S., Electrochemistry of Pitting Corrosion, 1 Pit Formation During Spontaneous Dissolution of Stainless Steels , Zashch. Metal.,1, 134(1966) C.A.,65, 10114a... 

Vetter, K. J. and Strehblow, H. H., Formation and Shape of Pitting Corrosion Pits in Iron and Theoretical Conclusions on Pitting Corrosion , Ber. Bunsenges Phys. Chem., 74, 1024 (1970) C.A., 74, 8865j... 

Use of material with good pitting corrosion resistance is desirable in seawater and oilfleld brine (formation water) media. 

Pitting the formation of holes or pits in a metal surface, by corrosion. 

Pitting (pitting corrosion) the formation of small holes in an otherwise passive metal surface as a consequence of locally accelerated corrosion. 

Strong Alcoholic Beverages. Products such as whiskey, cognac, brandy, etc. cause undesired reactions with unprotected aluminum. The attack causes pitting corrosion and formation of a floculent precipitate of aluminum hydroxide while the beverage itself becomes discolored, and the flavor is also affected (22). The action of liqueurs is not so... 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

Figure 49. Flow chart to compute the pit pattern-formation process." (Reprinted from M. Asanuma and R. Aogald, Morphological pattern formation in pitting corrosion, J. Electroanal. Chem. 396, 241,1995, Fig. 6 Copyright 1995, reproduced with permission from Elsevier Science.)...

These detailed microscopic studies show that it is possible to predict how and where pitting corrosion will occur on the surface. Like the titanium surface, an aluminum surface is passivated at normal temperatures by formation of an oxide layer in the ambient atmosphere. Despite formation of an oxide layer, aluminum surfaces can also be studied by STM. Pitting corrosion can be observed after 10 h of immersion of an aluminum surface at -1.2 V/normal hydrogen electrode in a IO-2 A/ NaCl electrolyte. The pitting on aluminum is observed as a general roughening... 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

The pitting corrosion of copper has been inhibited by injection of ferrous salt solution. Ferrous ion injection probably results in the formation of lepidrocrocite, FeO.OH, and provides protection. 

Fig. 20M Schematic representation of early stages in the formation of a pit. (a) the reactions taking place in and around a pit. (6) formation of a deposit of corrosion product, partially blocking the e.rit of the pit. (c) and (d) propagation of the pit, which is almost filled with solid corrosion products.

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

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