Danube University of GalatiFaculty of EngineeringSustainable development and security industry

Project: Effect of galvanic corrosion.Hydrogen induced cracking.

Student Andone Robert Supervisor Lidia Benea

Contents:

1. Introduction2. Definition3. Material factors3.1. Effects of coupled materials3.2. Effect of area3.3. Effect of surface condition4. Environmental factors4.1. Effects of solution5. Preventive measures6. Beneficial effects of galvanic corrosion7. Hydrogen induced cracking8. Hydrogen induced cracking detection

Introduction

Corrosion is the deterioration of a material, which results from a reaction with its environment. This environment comprises the physical, chemical and mechanical conditions or surroundings of the material. The corrosion of ferrous metals in ground water may be caused by electrochemical or physical processes. The various types of corrosion that can be generated from contact between water well components and ground water include electrochemical, crevice, and galvanic corrosion as well as stray electrical current or microbial induced corrosion. Physical processes may also corrode metals through fluid or particle impact. the material.

Galvanic corrosion

• Galvanic corrosion is either a chemical or an electrochemical corrosion. The latter is due to a potential difference between two different metals connected through a circuit for current flow to occur from more active metal (more negative potential) to the more noble metal (more positive potential)

• Galvanic coupling is a galvanic cell in which the anode is the less corrosion resistant metal than the cathode.

Galvanic corrosion can be predicted by using the electromotive force (emf) or standard potential series for metal reduction listed in Table at the next slide

In selecting two metals or two alloys for a galvanic coupling, both metals should have similar potentials or be close to each other in the series in order to suppress galvanic corrosion

As listed in Figure 1, all the factors affecting the electrode properties, such as those under categories (a)–(g), have an influence on galvanic action between any two metals.For example, titanium has a very negative reversible electrode potential and has an active position in the emf series. However, titanium occupies a noble position in the galvanic series in many practical environments due to passivation of the surface.

Effects of Coupled Materials

Galvanic coupling can be used for cathodic protection purposes

Other types of galvanic coupling are batteries and fuel cells

• Lithium Ambient-Temperature Batteries (LAMBS)

• Lead-Acid Battery• Dry-Cell Battery• Sintered Nickel Electrode in Alkaline

Batteries

MICROSTRUCTURAL EFFECTSA mechanically deformed metal or alloy can experience galvanic corrosion due todifferences in atomic plane distortion and a high dislocation density

Effect of Area The effect of anode and cathode areas on galvanic corrosion depends on the type of control in the system. If the galvanic system is under cathodic control, variation in the anode area has little effect on the total rate of corrosion, but variation of the cathode area has a significant effect. The opposite is true if the system is under anodic control. Galvanic currents in many situations are proportional to the surface area of the cathode .

FIGURE 2. Effect of area of mild steel cathode on weight loss of Zn anode (area of 100 cm2) and on number of coulombs flowing between Zn–steel couple over a 96-h period in 1N NaCl solution at 25°C

Effect of Surface Condition• The surface of metals in contact with an

electrolyte is gene- rally not “bare” but is covered with a surface layer, at least an adsorption layer, but often a solid surface film.

• This is the most important factor that causes the difference between the intrinsic polarity and apparent polarity and between the difference in potentials and the extent of galvanic corrosion.

• Formation of a surface film, whether a salt film or an oxide film, may significantly change the electrochemical properties of the metal surfaces, resulting in very different galvanic action.

ENVIRONMENTAL FACTORS

A corrosive environment is characterized by its physical and chemical nature, which may affect the electrochemical properties. Given that the electrochemical properties of each metal are distinctive in a given electrolyte, galvanic corrosion is essentially unique for each metal couple in each environment. The combination of metal couples and environmental conditions is, thus, limitless, as can be appreciated from Table 1.

PREVENTIVE MEASURES The essential condition for galvanic corrosion to occur is two dissimilar metals that are both electrically and electrolytically connected. Theoretically, prevention of galvanic corrosion can be achieved by avoiding the use of dissimilar metals in an assembly, by electrically separating the dissimilar metals with an insulating material or by physically insulating the environment from the metal surface with a coating impermeable to water. In reality, however, complete prevention is often not practical, as dissimilar metals need often to be used in direct contact and exposed to a corrosive environment and there is no absolutely impermeable coating. Thus, measures to minimize the possibility and extent of galvanic corrosion must be implemented All the factors listed in Figure .1 can be considered and controlled in order to reduce galvanic corrosion.

Some practical approaches are as follows:

• (a) Avoid combinations of dissimilar metals that are far apart in the galvanic series applicable to the environment.

• (b) Avoid situations with small anodes and large cathodes.

• (c) Isolate the coupled metals from the environment.• (d) Reduce the aggressiveness of the environment by

adding inhibitors.• (e) Use cathodic protection of the bimetallic couple

with a rectifier or a sacrificial anode.

BENEFICIAL EFFECTS OF GALVANIC

CORROSION

• As a result of galvanic corrosion of the anodic metal, the corrosion of the cathodic, coupled metal or alloy is generally reduced (i.e., cathodically protected).

• This effect has been well utilized in the application of sacrificial anodes, coatings, and paints for corrosion protection of many metal components and structures in various environments.

• Sacrificial anodes, mainly made of zinc, aluminum, and magnesium and their alloys, are widely used in corrosion prevention underwater and underground for structures such as pipelines, tanks, bridges, and ships.

Hydrogen induced cracking

• Hydrogen-induced cracking (HIC) refers to the internal cracks brought about by material trapped in budding hydrogen atoms. It involves atomic hydrogen, which is the smallest atom, that diffuses into a metallic structure. In the case of a crystal lattice becoming saturated or coming into contact with atomic hydrogen, many alloys and metals may lose their mechanical properties.

• In case the buildup of molecular H is repressed, the emerging atomic H can disperse into the metal rather than forming a gaseous reaction. This, in turn, produces a crack in the metal or material. Certain chemical elements may contribute to this like selenium, antimony, arsenic and cyanides. However, the topmost species is H2S, or hydrogen sulfide.

HYDROGEN INDUCED CRACKING DETECTION

• CrHydrogen Induced acking (HIC) is a failure mechanism resulting in sudden exposures and cracks due to growing laminations inside the base material and welds.

• Hydrogen Induced Cracking is a mechanical fracture caused by penetration and diffusion of atomic hydrogen into the internal structure of steel, which changes into molecular hydrogen at internal interfaces between non-metallic inclusions and the base material.

APPLICATIONS• Pipelines, towers, vessels, heat exchangers in aqueous sulphide environment of oil industry facilities, petrochemical plants and oil transportation• Parts where HIC has occurred and accurate testing is needed• Possible testing geometries: pipe, curved pipe, spherical surface and flat plate (diameter > 150 mm)

ADVANTAGES• Very high resolution equipment giving clear data presentation• Test speed: 150 mm/sec•Scanner accuracy: Maximum ±0.5 mm• Possibility to print an image of real defect size• Quantitative analysis, permanent integrity of data, periodic monitoring and side by side analysis• Identifies the need to apply further NDT to confirm or increase integrity