Cathodic Protection

The principle behind cathodic protection is as simple as it is effective. Cathodic protection moves the anodic oxidation reaction away from the steel surface and on to an external anode. The steel surface is transformed into the cathode in the electrochemical system and supports only cathodic reduction reactions. To achieve this the surface of the steel is flooded with electrons to suppress the anodic oxidation reaction. Two methods are available to achieve this and they are known as the Sacrificial Anode method and the Impressed Current method.

Chapter 1 - Introduction and Background to Fatigue of Tubular Joints

In the sacrificial anode method, a metal less noble (more negative) than steel is placed in electrical contact with the steel in the aqueous environment. Aluminium, zinc or magnesium are suitable materials. These metals will corrode in preference to the steel and supply electrons to support the cathodic reactions on the steel surface. The impressed current method uses an external power source to lower the potential of the steel surface. Noble metals are often used as the anode in impressed current systems, sustaining anodic reactions other than the dissolution of metal ions. This has the advantage that the anode is not consumed and does not need to be replaced. The impressed current CP system is shown schematically in Figure 1.39.

At the negative (electron rich) steel surface two reduction reactions are possible in sea water, namely the reduction of dissolved oxygen (1.19)

O2 + 2H2O + 4e' 4 0 H (1.19)

and the reduction of water (1.2 0)

H 2O + e —>Had.sorbcd + O H ( 1 . 2 0 )

It is the reduction of water reaction that is of most significance here as the hydrogen atoms can diffuse through the steel lattice structure. Hydrogen is known to promote brittle behaviour of normally ductile materials.

Discussion of the effect of cathodic protection an the fatigue crack growth process can be divided into two distinct topics:

a) Effects due to hydrogen

b) Effects of precipitated mineral deposits (calcareous deposits)

(i) Hydrogen based effects

The production of hydrogen as a by product of cathodic protection has already been discussed. Significant evidence exists to suggest that interstitially absorbed hydrogen can adversely alter the ductility of some steels. High strength steels are generally thought to be more susceptible than lower strength grades.

Loss of ductility due to the presence of hydrogen is commonly termed hydrogen

embrittlement. The term “hydrogen embrittlement” is used to cover a wide range of observations where unusual material behaviour is observed where hydrogen gas may be present. The effects of hydrogen are generally seen in parameters such as elongation and reduction of area in tensile tests and fracture toughness values that are highly dependant on strain rate.

The exact mechanism by which hydrogen causes the degradation of material properties is not too clear. However it seems obvious from the literature that the mechanism involves the transportation of hydrogen to a tri-axially stressed crack tip or notch root. An increase in brittle fracture modes on the fracture surfaces of cathodicaUy protected specimens helps to confirm hydrogen embrittlement as the mechanism responsible for increased growth rates under CP conditions [1.81,

1.82]

A discussion on the theories behind hydrogen embrittlement is given by Cottis [1.83] who notes that five main theories have been proposed to account for the observed effect of hydrogen on high tensile steels.

(i) Pressure Theory.

Chapter 1 - Introduction and Background to Fatigue of Tubular Joints

to imagine the formation of high pressure pockets of gas suggesting that some other mechanism may be (additionally) operative.

(ii) Decohesion Theory.

(iii)Surface Energy Theory.

(iv) Hydride Formation Theory.

(v) Local Plasticity Theory.

Although experimental evidence exists to support each of the theories, it is thought that the effect of hydrogen can be distilled down to the following:[1.83]

1) Hydrogen can decrease the strength of the metal - metal bond thus facilitating brittle fracture.

2) Hydrogen can increase the stress required to emit dislocations from the crack tip, making ductile failure more difficult.

Anodic dissolution has been shown to be responsible for the increased crack growth rates under free corrosion conditions. This mechanism is not thought to be significant in the case of cathodic protection since the oxidation reactions responsible for the dissolution of iron ions should be suppressed at the potentials under consideration here. Measurement of the potential at the crack tip of actual and simulated cracks has shown that polarisation of the specimen is virtually as

effective at changing the potential within the crack as upon the external surface [1.85].

The simultaneous application of mechanical strain and hydrogen charging appears to be essential to the mechanism of hydrogen embrittlement. This is shown by slow strain rate tensile tests in air using specimens which had previously been allowed to soak under CP conditions in sea water [1.86]. After 100 hours exposure the specimens were removed from the sea water and subjected to a tensile test in air. The results show no loss of ductility occurs when the straining and hydrogen charging are not simultaneous.

The dynamic strain rate is clearly an important variable in determining the potential susceptibility to fatigue crack growth rate enhancement under cathodic protection conditions. The effect of strain rate has been investigated by Proctor [1.87] who performed tensile tests on X65 Linepipe steel in 3.5% NaCl solution at very negative levels of CP. The results show increasingly ductile behaviour as the strain rate increases. This has implications for the waveform used during testing with the rise time being of particular importance. This has been demonstrated by examining the crack growth rates of specimens subjected to square, sinusoidal and triangular waveforms in 3.5% NaCl solution [1.88]. The triangular and sinusoidal growth rates were similar to each other and consistently faster than the average of the air data . However the square waveform resulted in a significantly lower growth rate, only marginally faster than in air. This can also be translated into an effect of frequency with the effect of environment likely to decrease with increasing frequency. Atkinson and Lindley [1.89] came to much the same conclusion using triangular and positive and negative saw tooth waveforms. It is postulated that during dynamic straining new material is being exposed to the environment via the disruption of passivating layers allowing the dissolution of iron ions to take place in free corrosion and the adsorption of hydrogen under CP conditions. The passivating layers are therefore assumed to reform rapidly once the peak load in each cycle has been reached.

Chapter 1 - Introduction and Background to Fatigue of Tubular Joints

Naturally, any mechanism that encourages the absorbtion of adsorbed hydrogen (rather than harmlessly bubbling away as a gas) is likely to magnify the measured effects of hydrogen embrittlement. Sulphate Reducing Bacteria, common in natural sea water environments are known to promote the absorbtion of hydrogen. Cowling et al have studied the role of SRB extensively for 50D type steels [1.90].

The promotion of corrosion and corrosion fatigue by SRB’s is said to be due to the following processes:

1) The ease with which SRB’s reduce sulphate ions to sulphide ions which rapidly hydrolyse to form hydrogen sulphide.

2 ) T h e u s e b y th e b a c te r ia o f h y d r o g e n as a n e n e r g y s o u rc e , th u s p r o d u c in g c a t h o d ic d e p o la r is a t io n ( p r o m o t in g c o iT O s io n ).

3) Enhanced hydrogen embrittlement due to increased permeation of atomic hydrogen into bulk metal.

Tests were performed on 25mm thick, three point bend specimens at a CP level of -850mV. Enhanced corrosion fatigue crack growth rates occur across a limited range of AK and is noted as being increased by up to an order of magnitude. This is the same region where enhanced crack growth rates due to cathodic protection are found. Robinson and KilgaUon [1.91] have investigated the effect of SRB’s on hydrogen damage in high strength steels. The general conclusion that SRB’s considerably increased the level of absorbed hydrogen and thus the possibihty of increased embrittlement was confirmed.

(ii) Calcareous Deposits

When a steel surface is cathodicaUy polarised in a natural or artificial sea water, calcium and magnesium based mineral deposits can form on the surface of the steel [1.92]. Oxygen reduction reactions at the steel surface increase the pH of the solution adjacent to the steel. Other reactions also occur which have the same

effect. The increased alkalinity at the surface impedes the entry of into the metal hydrogen by affecting the reduction reactions at the surface thus limiting the effect of hydrogen embrittlement [1.93].

A further effect of this layer of more alkaline sea water adjacent to the steel is to promote the following precipitation reactions:

O H - + H C O j - - 4 H2O + C O j^ - ( 1 . 2 1 )

ppt

CO3" + ^ CaCOj (1.22)

CGj^' + M g '^ ^ M g C G jrp , (1.23) 2GH + Mg'+ Mg(GH)2 pp, ( 1.24)

Hodgkeiss et al [1.85] and Maahn [1.94] have both investigated the pH of the solution within a fatigue crack. Both investigations noted that the pH within the crack is commonly more alkaline than the pH of the bulk solution. The pH inside the crack is typically between 10 and 13. This has been confirmed by the formation of M g(OH)2 within the confines of cathodicaUy polarised fatigue cracks.

Magnesium hydroxide does not form at pH more acidic than 10. Magnesium hydroxide has been shown to form at a faster rate than calcium carbonate [1.95] and results in a harder and stronger precipitate.

Formation of these precipitates, known as ‘calcareous deposits’ within a fatigue crack has been shown to reduce the effective SIF range by wedging the crack open at the lower loads in the fatigue cycle. This precipitate induced crack closure is known to increase the minimum value of crack opening in a cycle whilst leaving the maximum unchanged.

It is also thought that the calcareous deposits on the surface of the steel may inhibit the entry of hydrogen into the steel by restricting the supply of water at the steel surface therefore slowing the rate of water reduction and hydrogen evolution.

C hapter 1 - Introduction and Background to Fatigue of Tubular Joints