Seawater corrosion testing pitting

FIG. 1—Sequence of events (left to right) in a stress corrosion test on an iniUatty smooth specimen. For low-alloy steels In seawater, the rale of growth of SCC is faster than it is for pitting by a factor of about 106, and fast fracture propagates at about 1010 times faster than SCC [0]. 

Frequent use has been made of ASTM G 48, Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys fy the Use of Ferric Chloride Solution. Specimens are immersed in ferric chloride or acidified ferric chloride and are evaluated by visual examination and mass loss. A related document. Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments is found in ASTM G 78. 

Numerous tests - including natural seawater exposure tests - have demonstrated that a small chromium addition reduces the corrosion rates considerably without rendering steels more susceptible to pitting corrosion. In the upper part of Figure 15, the influence of chromium on seawater corrosion of a structural steel is presented [47]. Accordingly, only 0.5% and 1% Cr have a significant effect and reduce mass losses by 35%/65% compared to chromium-free steel. Improvements from higher chromium contents above this level are then relatively small. 

Table 3.23 Effect of seawater velocity on pitting corrosion of 316S3I. Tests for 3 /z years...

Comparative tests between HSI and HSCI in seawater at 93° C and 10-8Am showed consumption rates of 8-4kg A y and 0-43 kg A y , respectively . These figures show that the consumption rate of HSI when used in seawater without the addition of chromium may approach that of steel, but because of the very deep pitting and its fragility, it is in most cases inferior to steel. However, in fresh waters HSI has a far lower corrosion rate than steel. The consumption rate of HSCI freely suspended in seawater in the current density range 10-8 to 53-8 Am increases from 0-33 kg A y at 10-8Am to 0-48 kg A" y at 53-8Am Direct burial in seawater silt or mud will also increase the consumption rate, with values of 0-7kg A y at 8-5 Am increasing to 0-94 kg A " y at 23-4 Am . 

As noted in Table 3 [77], the results of multiple alloy tests in seawater are correlated to the Pitting Resistance Equivalence (PRE) number [47,48,72,77], which also correlates to alloy pterformance in FeCly. Anderson [78] and Streicher [79] used MCA in seawater tests to compare alloy performance. More recently, a simple, flat, plastic (specifically, polymethylmethacrylate, which is often referred to as "perspex ) washer has been successfully used to evaluate a series of alloys in seawater [77]. One application of the ASTM G 48 test has been in simulating leaking tube-to-tube sheet joints in seawater heat exchangers and condensers [87]. When certain highly corrosion-resistant alloys were paired in a dissimilar metal crevice (DMC) with alloys that would be expected to suffer crevice corrosion in the particular test solution, the more corrosion-resistant alloy was found to corrode due to the accelerating effects of the corrosion products from the less resistant alloy. The results of DMC tests in ferric chloride were confirmed by long-term DMC exposures in seawater [82],... 

Tests of ferritic CrMo steels show that pitting corrosion is suppressed in seawater and crevice corrosion can be reduced to low levels or long incubation periods at chromium contents of 25% and molybdenum contents of at least 3% [103,104]. 

In stress corrosion cracking tests with U-bend samples in boiling synthetic seawater, the nickel-free steels also remained free of stress corrosion cracking after a test period of 1,200 h, but they did show pronounced shallow pit and crevice corrosion. Nickeliferous samples were also resistant to stress corrosion cracking, and also showed no signs of local corrosion under these conditions [110]. 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

The martensitic tempering steel X4CrNiMol6-5-l (DIN-Mat. No. 1.4418) has proved resistant to stress corrosion cracking in stress corrosion cracking tests in seawater up to 333 K (60 °C) and stress loads of up to 90% of the 0.2% yield point, but did show pitting corrosion [117]. 

Long-term studies of condenser pipes made of the austenitic steels (DIN-Mat. No. 1.4438, X2CrNiMol8-5-4, 1.4439, X2CrNiMoN17-13-5 and 1.4558, X2NiCrAlTi32-20) under quasi-industrial conditions of thermal seawater desalination confirm that the pitting resistance equivalent of these steels increases with increasing molybdenum content, but also that a content level of 4.16% Mo was not sufficient to prevent pitting corrosion under the test conditions. In this test in brine 3.5-10% salt content, a pH level of 6.5-7 and a temperature of max. 401 K (128°C), only the pipes made of titanium showed complete resistance [140]. 

The standard steels of the type SAE 316 (DIN-Mat. No. 1.4401, X5CrNiMol7-12-2) are not suitable for seawater-exposed pipes and fail as a result of pitting and crevice corrosion [155, 156]. The sensitivity to pitting corrosion of these standard steels can be further increased by deposits of maritime bacterial films [157]. Despite these facts, these steels are frequently used as materials for pump parts and have worked well as such because they are cathodically protected by contact with other parts made of less noble materials, e.g. pump casing made of cast iron [130]. [158] reports on tests of the cavitation behaviour of the pump materials GX5CrNiMol9-ll-2 (DIN-Mat. No. 1.4408) in 3% NaQ solution. 

A test alloy based on X4CrNiMoN19-16 with a raised Cr content of 23.42% and a nitrogen content of 0.41% shows a high pitting resistance equivalent as well as satisfactory resistance to crevice corrosion in seawater at ambient temperatures [165]. 

In tropical waters, the corrosion values for nickel are higher than in the temperate climatic zones. The pitting depths reach approx. 3 mm after only 1 year, after which the penetration rate drops. Figure 43 and Figure 44 show the results of exposure tests of nickel and nickel alloys in the seawater of the Panama Canal Zone. Whereas in the immersion zone pitting depths of over 3 mm were reached, the values in the tidal zone were about 1.6 mm [192]. 

In either the solution treated or aged condition, it is corrosion resistant to seawater, salt emd other natural environments, oxidizing media, inhibited reducing acids, alkalies, and metallic chlorides at room temperature. In salt-spray tests, aged Ti-13V-llCr-3Al exhibits no pitting and experiences no general corrosion or degradation in mechanical... 

The long-term-exposure tests indicated that the rolled surfaces of the 8090-T851 sheet were more resistant to corrosion than those of the conventional 2024-T3 sheet. Except for some pits that developed at an air/water interface, these surfaces suffered only minor corrosion. The same tests indicated that the rolled surfaces of the 2090-T8 sheet suffered at least as much corrosion damage as their counterparts on the 7075-T6 sheet. Some fairly deep pits occurred on the rolled surfaces of the 2090, even during the exposure to seawater fog. 

The tested aluminium alloys have very good corrosion resistance in contact with surface seawater. For example, the extrapolated decrease in thickness after 30 years would be on the order of 200 xm for alloys 3004 and 5052. In contact with deep seawater, the same alloys exhibit pitting corrosion with a depth up to 200 p.m after 3 months of operation [33]. 

---