Preferential weld corrosion: Effects of weldment microstructure and composition

C-M Lee, S Bond and P Woollin

Paper presented at NACE 2005 Houston, Texas, 3-7 April 2005

Abstract

Preferential weldment corrosion (PWC) of carbon and low alloy steels used for pipelines and process piping systems in CO ₂ -containing media has been observed increasingly in recent years. In particular, this has been on weldments made by the manual metal arc (MMA) process using electrodes containing Ni or Ni plus Cu. This paperpresents the results of a joint industry research programme which was conducted collaboratively by three research organisations to investigate this corrosion mechanism and to seek practical solutions.

The effect of composition and microstructure on PWC in CO ₂ -containing media was investigated on 12 weldments produced in X52 and X65 grade pipe materials using TIG and MMA processes. Corrosion tests were conducted in a re-circulating vessel on segmented weld electrodesin CO ₂ -containing media, with two levels of chloride content. The addition of increased amounts of nickel and silicon was detrimental, whilst additions of molybdenum and chromium (of up to 0.7wt%) did not giveimprovements in PWC behaviour. Autogenous weldments, made without filler additions, and weldments made with matching composition consumables gave the best PWC resistance. It is also shown that empirical relationships exist between PWCand hardness levels and microstructure, with unrefined microstructures, having high hardness, being detrimental. The implications of the data for design of welding procedures to minimise PWC are considered.

Introduction

Preferential weldment corrosion (PWC) of carbon and low alloy steels used for pipelines and process piping systems in CO ₂ -containing media has been observed increasingly in recent years. Attempts to control PWC have previously involved making minor additions of noble metals (e.g. Ni, Cr, Mo, Cu) to the weld consumable in order tomake the weld metal cathodic with respect to the adjacent parent pipe and HAZ. This approach has proved successful in preventing preferential weld attack in seawater injection systems where additions of either Ni or Ni and Cu can beused to prevent preferential corrosion providing care is taken to avoid over-alloying, which can induce attack of the HAZ ^[1] .

The use of nickel-containing weld consumables has also been widely adopted in production systems. However, while some companies have satisfactory experience of using nickel-containing consumables, there have been examples of severepreferential attack of the weld metal in sweet environments ^[2] .

Studies have shown that PWC in CO ₂ -containing media is influenced by a complex interaction of several parameters including the environment, flow conditions, scaling effects, parent steel composition and welding procedure ^[3-5] . No general agreement exists on the role of alloying elements and microstructure in preferential corrosion of welds ^[3] .

A recently completed joint industry project investigated the PWC of ferritic steels in CO ₂ -containing environments. This paper reports the findings on the effects of weld microstructure and composition on PWC from the research programme. Two other papers, published in this conference, will focus oncorrosion flow loop studies of PWC and its inhibition in CO ₂ environments ^[6] and PWC of 1%Ni welds as a function of solution conductivity and inhibition ^[7] . Based on the findings of the project a set of guidelines for the prediction, control and monitoring of PWC of ferritic steels has recently been published ^[8] .

Experimental method

Welding

Three parent pipe materials were used on the project. These were:

• API 5L X52/ASTM A333 grade 6, 356mm diameter, 25mm wall thickness.
• API 5L X65, 508mm diameter, 10.3mm wall thickness.
• API 5L X60 (with 0.5%Cr addition), 273mm diameter, 12mm wall thickness.

The chemical analyses of these pipes are given in Table 1. Welding was carried out by two processes, namely manual metal arc (MMA) and automated cold wire tungsten inert gas (TIG) welding to produce 12 different weld compositions and microstructures. Four elements, Ni, Cr, Moand Si, were introduced into the weld metal in small amounts using commercially available consumables. The two welding processes were used to achieve low and high levels of dilution in welds containing Ni and Cr. Welds made withnominally matching consumables and autogenous welds were also included for comparison. Details of the welding consumables and significant alloying elements are given in Table 2.

Table 1 - Chemical analysis of the parent pipe materials

As <0.004, except X52 for which As=0.024; Co ≤ 0.011; Pb, Zr <0.005; Sn ≤ 0.010; Ti ≤ 0.003, except X65, for which Ti = 0.042; W <0.01; Ce <0.02; Sb <0.002; B, Ca <0.0003.

Table 2 - weld details

Corrosion Tests

Test Vessel. The design of the test vessel is shown in Figure 1; the cylindrical body of the main vessel was made from titanium. A paddle was driven from a motor mounted beneath the vessel to provide circulation. The test vessel was connected to a glass reservoir to provideadditional solution volume and pumps were used to circulate the solution through the oxygen monitoring equipment.

Sample Preparation.

Test Parameters.

Table 3 - Corrosion test details

Test Procedure.

The oxygen level of the test solution was monitored using a dissolved oxygen meter and maintained <10ppb. The pH and Fe ²⁺ ion concentration were also monitored on samples taken periodically from the test solution. The Fe ²⁺ were measured using atomic adsorption spectrometry. On completion of the test after 30 days, the specimens were removed, and rinsed with distilled water and ethanol.

The electrochemical measurements were carried out using a computer controlled potentiostat with a weld test facility on each of the five segments (i.e. weld metal, 2 x HAZ, 2 x parent). Linear Polarisation Resistance (LPR)measurements was used for calculating corrosion rates in the high conductivity solution and Electrochemical Impedance Spectroscopy (EIS) was used in the low conductivity solution. Details of the corrosion rate calculation methods aregiven in one of the associated papers ^[7] .

Results and discussion

Weld Characterisation

Compositionally, the 12 welds produced can be divided into 6 main categories; welds containing Ni, Cr, Si and Mo alloying additions, welds made with nominally matching filler and autogenous welds. Table 4 shows results of hardness and grain size measurements of the 12 welds tested in the programme.

Table 4 - characterisation of weld metal microstructures

The deposition of welds with the two welding processes and a range of welding consumables produced weld root beads with a range of microstructures. Most of the welds had root microstructures that had been largely refined to equiaxedferrite and pearlite. However two, W21 and W25, consisted predominately of ferrite with aligned 2 nd

The microstructures were each assigned ratings in two categories; firstly the degree of refinement of the microstructure towards one of equiaxed ferrite and pearlite, labelled R, and secondly the extent of ferrite with alignedsecond phase, labelled A. A value of 0 to 3 was assigned in each category, to represent the visually assessed extent of refinement and aligned second phase present. R=0 showed no refinement to equiaxed ferrite and pearlite, R=3represented a fully refined structure of equiaxed ferrite and pearlite. A=0 indicated little or no ferrite with aligned second phase, whilst A=3 indicated the highest level of aligned second phase observed. Table 4 also shows the assigned ratings of each of the 12 weldments.

Corrosion Tests

Current Measurements vs Time. The galvanic current measurements from selected samples are shown in Figures 3 and 4. Note that the y-axis scales for the graphs are not the same for low and high chloride tests due to a large difference in the magnitude of the measured currents. In general, the magnitude of the current was the highest forwelds containing alloying elements. When reduced corrosion rates arose from film formation, towards the end of the test, the galvanic currents followed a similar falling trend.

In Ni-containing weld samples (W20 and W26), the weld metal showed a consistent cathodic behaviour with respect to the other segments, in low and high chloride tests, Figure 3

Corrosion Rates vs Time. Figures 5 and 6 show calculated corrosion rates from selected samples from low chloride (Test 4) and high chloride (Test 3) tests. Typically, the weld metal shows preferential corrosion in the early part of the test and then falls to alevel nearer to that of the parent steel. After the initial peak, the falling corrosion rates are attributed to changes in the metal surface condition. In the period from 5 to 10 days, the metal surfaces are relatively free from asurface film, so they corrode relatively freely in the solution. After this period, as Fe ²⁺ ions build up in the solution, some segments of the weldment samples begin to form a protective scale on the surface and corrosion rates begin to drop dramatically. This effect is typically seen first in theweld metal. This is followed by the preferential corrosion of the HAZ which shows the next highest corrosion rates and these segments subsequently form a protective scale and also typically show reduced corrosion rate towards the endof the test. This result explains the occurrence of preferential HAZ attack rather than preferential weld metal attack, as observed in some field failures.

Not all samples and segments showed a reduction in corrosion rate during the test. This indicates that a build-up of protective scale did not occur in these segments. This is confirmed by visual examination of samples after thetest, where certain segments were either partially covered with the protective film or the film formed was loosely adherent to the surface and so unable to provide complete protection.

Effects of Chloride Concentration. Figure 7 shows the average calculated EIS corrosion rates between 5-10 days from the low chloride test and Figure 8 shows the peak LPR corrosion rates from the high chloride test. These values were chosen to be representative of the freely corroding period before film formation and after the initial transient.

With the exception of the Ni and Si-containing weld metal segments in the low chloride test, the corrosion rates of all parts of the weldment are generally greater in the high chloride test. However, the difference between thecorrosion rates of the weld metal and parent steel, i.e. the degree of preferential corrosion, is greater for the low chloride solution test. For all weldments, it was noted that preferential weld metal/HAZ corrosion was lower in thehigh chloride environment compared to the low chloride.

Effects of Composition. Figures 7 and 8 show that the weld metals made with matching composition consumables or without consumable addition gave lower corrosion rates and better resistance to preferential weld metal attack than those made with alloyed consumables,with Ni being particularly detrimental. The data indicate that silicon has a detrimental effect on preferential weld metal corrosion also but this effect was less than that for nickel. It is noted that in another study on parentmaterial, silicon was shown to have a beneficial effect on CO ₂ corrosion resistance at temperatures greater than 93°C ⁹ . Molybdenum addition at the level examined had little or no effect on preferential weld metal corrosion although other workers have found some beneficial effects ^[2,10] . The data for chromium-containing weld metals were inconclusive. However the data did not rule out the possibility that there might be some beneficial effect of higher levels of Cr, as would be expected intuitively. Overall thebest performance with regard to reducing preferential weld metal corrosion was obtained with matching composition or autogenous weld metals. However, preferential weld metal corrosion was not avoided for any of the weld metalsexamined. Figure 9 shows an example of a 0.6wt%Ni-containing weld metal that was particularly prone to PWC, where compositional effects dominate over microstructure.

Nevertheless chromium is known to be beneficial, in the parent material, if present in sufficient levels [10,11]

Influence of Microstructure. The root weld metal microstructure classification is given in Table 4. It was found that there was a reasonable correlation between microstructure and preferential weld metal corrosion for weld metals with no significant deliberate alloying (i.e. welds made with matching compositionconsumables and autogenous welds). Weld metals having largely unrefined ferrite with aligned second phase root microstructures (low R, high A), tended to give preferential weld metal corrosion. Refined weld metals with less alignedsecond phase (high R, low A) tended to have similar corrosion rates to the parent steel, except where a compositional effect, in particular nickel addition, had an overriding detrimental influence, in particular in the low chloridesolution.

Further correlations were developed, relating the peak corrosion rate of the weld metal in each sample to the root weld metal hardness and grain size. Figures 10-13 show plots of these correlations. The plots show a modest trend of increasing corrosion rate with increasing hardness and increasing grain size for weld metal without significant alloying additions, consistentwith the visual microstructure assessments. Once again, the addition of Ni, and to a lesser extent Si, tends to give higher corrosion rate for a given hardness level or grain size in the low chloride tests. The effects of Cr and Mowere small but not consistently beneficial or detrimental. Figure 14 shows an example of a weld metal microstructure that promotes PWC where there is no over-riding composition effect.

Fig.14. Unalloyed weld metal with a fairly coarse prior austenite grain size and a high level of aligned second phase. Weld was deposited in X52 pipe with the TIG process and an ER70S-6 wire, W21. The microstructure promotes PWC but the composition does not

In some cases preferential HAZ corrosion increased with increasing HAZ hardness, consistent with the weld metal data. This suggests that pipe with low carbon equivalent therefore might be preferred. However, the trend was notobserved consistently. This probably reflects interaction with the weld metal, i.e. preferential attack of weld metal may tend to discourage preferential HAZ attack and vice versa.

Conclusions

A strong link between preferential weld metal corrosion and composition was found, particularly in a low chloride solution, although preferential weld metal corrosion cannot be prevented solely by the use of an alternative weldmetal composition selected from those studied. Greatest resistance to preferential weld metal corrosion was obtained for autogenous root deposits or for welds deposited using consumables without significant alloying addition. Additionof 1% nickel was detrimental, as was 1% silicon. Addition of 0.5% molybdenum or 0.6-0.7% chromium to the weld metal had no consistent beneficial effect with respect to preferential weld metal corrosion.

Preferential weld metal corrosion increased with increasing hardness, increasing grain size, an increasing level of aligned second phase and a decreasing level of microstructure refinement of the root by the subsequent passes.

Preferential HAZ corrosion occurred during test when film formation occurred on weld metal preferentially. Some evidence suggested that high HAZ hardness might encourage preferential HAZ corrosion but this was not conclusive.

Welds made by TIG and MMA processes were compared but this was primarily to achieve different weld metal compositions rather than to allow a direct comparison of the welding processes. No distinct difference between the performanceof welds made by the two processes, with respect to preferential weld corrosion, was found.

Acknowledgement

This work was carried out in the Joint Industry Project 'Risk of Preferential Weldment Corrosion of Ferritic Steels in CO ₂ -Containing Environments', conducted 2000-2003 by TWI, CAPCIS, and Institute for Energy Technology (IFE). The project was sponsored by BP, ENI SpA, Health & Safety Executive (UK), Petrobras, Saudi Aramco,Shell UK Ltd, and Total Fina Elf.

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