دنیای متالورژی

The article analyses stainless steel resistance to burning products released by heat

burners. The research methodology as well as result findings are presented in the article as

well.

Ten years ago with the increase of fuel prices, upper combustion burners of solid fuel were being

replaced by lower combustion burners due to a longer burning cycle of loading and higher coefficient

of efficiency. The temperature of smoke, released from these types of burners is lower than 200 0C.

When the products of combustion cool down on the top of a chimney, water steam condenses [1, 2].

The condensate is made of various compounds, including sulfur as well. The saturated condensate

erodes the chimney and the soot that sticks in the place, where the condensate forms, blocks the

chimney. The saturated place of the chimney becomes more transcalent and the fume is cooled more

intensively; thus, the process speeds up and lower parts of the chimney saturate with the condensate as

well. These types of troubles could be avoided by fitting in a steel insert that weighs several kilograms.

The insert heats up very fast and the due point is passed over the period of 10-20 minutes as the air-gap

between the chimney and the insert contains quite a high resistance. However, the majority of stainless

steel is acid nonresistant; and the products of corrosion can include poisonous compounds of chromium

and nickel. In this article, we analyze the corrosion of chimney parts made of stainless steel under the

conditions of acid environment. Several types of stainless steel were examined.

Corrosion is deterioration of essential properties in a material due to reactions with its surroundings.

Millions of dollars are lost each year because of corrosion [3]. Much of this loss is due to the corrosion

of iron and steel, although many other metals may corrode as well. The problem with iron as well as

many other metals is that the oxide formed by oxidation does not firmly adhere to the surface of the

metal and flakes off easily causing "pitting". Extensive pitting eventually causes structural weakness

and disintegration of the metal.

Although one of the main reasons why stainless steels are used is corrosion resistance, they do

in fact suffer from certain types of corrosion in some environments and care must be taken to select a

grade which will be suitable for the application. Corrosion can cause a variety of problems, depending

on the applications:

• Perforation such as of tanks and pipes, which allows leakage of fluids or gases,

Corrosion Analysis of Stainless Steel 509

• Loss of strength where the cross section of structural members is reduced by corrosion,

leading to a loss of strength of the structure and subsequent failure,

• Degradation of appearance, where corrosion products or pitting can detract from a

decorative surface finish,

• Finally, corrosion can produce scale or rust which can contaminate the material being

handled; this particularly applies in the case of food processing equipment.

Corrosion of stainless steels can be categorized as one of [4]:

• General Corrosion

• Pitting Corrosion

• Crevice Corrosion

• Stress Corrosion Cracking

• Sulfide Stress Corrosion Cracking

• Intergranular Corrosion

• Galvanic Corrosion

• Contact Corrosion

Corrosion whereby there is a general uniform removal of material, by dissolution, e.g. when stainless

steel is used in chemical plant for containing strong acids. Design in this instance is based on published

data to predict the life of the component. Published data list the removal of metal over a year. Tables of

resistance to various chemicals are published by various organizations and a very large collection of

charts, lists, recommendations and technical papers are available though stainless steel manufacturers

and suppliers.

Under certain conditions, particularly involving high concentrations of chlorides (such as sodium

chloride in sea water), moderately high temperatures and exacerbated by low pH (i.e. acidic

conditions), very localized corrosion can occur leading to perforation of pipes and fittings etc. This is

not related to publish corrosion data as it is an extremely localized and severe corrosion which can

penetrate right through the cross section of the component. Grades high in chromium, and particularly

molybdenum and nitrogen, are more resistant to pitting corrosion.

The Pitting Resistance Equivalent number (PRE) has been found to give a good indication of the

pitting resistance of stainless steels. The PRE can be calculated as:

PRE = %Cr + 3.3 x %Mo + 16 x %N

One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong

tendency for it to continue to grow, even although the majority of the surrounding steel is still

untouched. The tendency for a particular steel to be attacked by pitting corrosion can be evaluated in

the laboratory. A number of standard tests have been devised, the most common of which is that given

in ASTM G48. A graph can be drawn giving the temperature at which pitting corrosion is likely to

occur, as shown in Figure 1.

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Figure 1: Temperature at which pitting corrosion is likely to occur

This is based on a standard ferric chloride laboratory test, but does predict outcomes in many

service conditions.

The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on

its surface, but it is possible under certain conditions for this oxide layer to break down, for example in

reducing acids, or in some types of combustion where the atmosphere is reducing. Areas where the

oxide layer can break down can also sometimes be the result of the way components are designed, for

example under gaskets, in sharp re-entrant corners or associated with incomplete weld penetration or

overlapping surfaces. These can all form crevices which can promote corrosion. To function as a

corrosion site, a crevice has to be of sufficient width to permit entry of the corrodent, but sufficiently

narrow to ensure that the corrodent remains stagnant. Accordingly crevice corrosion usually occurs in

gaps a few micrometers wide, and is not found in grooves or slots in which circulation of the corrodent

is possible. This problem can often be overcome by paying attention to the design of the component, in

particular to avoiding formation of crevices or at least keeping them as open as possible. Crevice

corrosion is a very similar mechanism to pitting corrosion; alloys resistant to one are generally resistant

to both. Crevice corrosion can be viewed as a more severe form of pitting corrosion as it will occur at

significantly lower temperatures than does pitting.

Under the combined effects of stress and certain corrosive environments stainless steels can be subject

to this very rapid and severe form of corrosion. The stresses must be tensile and can result from loads

applied in service, or stresses set up by the type of assembly e.g. interference fits of pins in holes, or

from residual stresses resulting from the method of fabrication such as cold working. The most

damaging environment is a solution of chlorides in water such as sea water, particularly at elevated

temperatures. As a consequence stainless steels are limited in their application for holding hot waters

(above about 50°C) containing even trace amounts of chlorides (more than a few parts per million).

This form of corrosion is only applicable to the austenitic group of steels and is related to the nickel

content. Grade 316 is not significantly more resistant to SCC than is 304. The duplex stainless steels

are much more resistant to SCC than are the austenitic grades, with grade 2205 being virtually immune

at temperatures up to about 150°C, and the super duplex grades are more resistant again. The ferritic

grades do not generally suffer from this problem at all. In some instances it has been found possible to

improve resistance to SCC by applying a compressive stress to the component at risk; this can be done

by shot penning the surface for instance. Another alternative is to ensure the product is free of tensile

stresses by annealing as a final operation. These solutions to the problem have been successful in some

cases, but need to be very carefully evaluated, as it may be very difficult to guarantee the absence of

residual or applied tensile stresses. From a practical standpoint, Grade 304 may be adequate under

Corrosion Analysis of Stainless Steel 511

certain conditions. For instance, Grade 304 is being used in water containing 100 - 300 parts per

million (ppm) chlorides at moderate temperatures. Trying to establish limits can be risky because

wet/dry conditions can concentrate chlorides and increase the probability of stress corrosion cracking.

The chloride content of seawater is about 2% (20,000 ppm). Seawater above 50°C is encountered in

applications such as heat exchangers for coastal power stations. Recently there have been a small

number of instances of chloride stress corrosion failures at lower temperatures than previously thought

possible. These have occurred in the warm, moist atmosphere above indoor chlorinated swimming

pools where stainless steel (generally Grade 316) fixtures are often used to suspend items such as

ventilation ducting. Temperatures as low as 30 to 40°C have been involved. There have also been

failures due to stress corrosion at higher temperatures with chloride levels as low as 10 ppm. This very

serious problem is not yet fully understood.

Of greatest importance to many users in the oil and gas industry is the material's resistance to Sulfide

stress corrosion cracking. The mechanism of SSC has not been defined unambiguously but involves

the conjoint action of chloride and hydrogen Sulfide, requires the presence of a tensile stress and has a

non-linear relationship with temperature. The three main factors are Stress Level, Environment and

Temperature.

A threshold stress can sometimes can be identified for each material - environment combination. Some

published data show a continuous fall of threshold stress with increasing H2S levels. To guard against

SSC NACE specification MR0175 for Sulfide environments limits the common austenitic grades to

22HRC maximum hardness [5].

The principal agents being chloride, hydrogen Sulfide and pH. There is synergism between these

effects, with an apparently inhibiting effect of Sulfide at high H2S levels.

With increasing temperature, the contribution of chloride increases but the effect of hydrogen

decreases due to its increased mobility in the ferrite matrix. The net result is a maximum susceptibility

in the region 60-100°C. A number of secondary factors have also been identified, including amount of

ferrite, surface condition, presence of cold work and heat tint at welds.

Intergranular corrosion is a form of relatively rapid and localized corrosion associated with a defective

microstructure known as carbide precipitation. When austenitic steels have been exposed for a period

of time in the range of approximately 425 to 850°C, or when the steel has been heated to higher

temperatures and allowed to cool through that temperature range at a relatively slow rate (such as

occurs after welding or air cooling after annealing), the chromium and carbon in the steel combine to

form chromium carbide particles along the grain boundaries throughout the steel. Formation of these

carbide particles in the grain boundaries depletes the surrounding metal of chromium and reduces its

corrosion resistance, allowing the steel to corrode preferentially along the grain boundaries. Steel in

this condition is said to be "sensitized". It should be noted that carbide precipitation depends upon

carbon content, temperature and time at temperature. The most critical temperature range is around

700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02%

carbon steels are effectively immune from this problem. It is possible to reclaim steel which suffers

from carbide precipitation by heating it above 1000°C, followed by water quenching to retain the

carbon and chromium in solution and so prevent the formation of carbides. Most structures which are

welded or heated cannot be given this heat treatment and therefore special grades of steel have been

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designed to avoid this problem. These are the stabilized grades 321 (stabilized with titanium) and 347

(stabilized with niobium). Titanium and niobium each have much higher affinities for carbon than

chromium and therefore titanium carbides, niobium carbides and tantalum carbides form instead of

chromium carbides, leaving the chromium in solution and ensuring full corrosion resistance [6].

Another method used to overcome intergranular corrosion is to use the extra low carbon grades

such as Grades 316L and 304L; these have extremely low carbon levels (generally less than 0.03%)

and are therefore considerably more resistant to the precipitation of carbide.

Many environments do not cause intergranular corrosion in sensitized austenitic stainless steels,

for example, glacial acetic acid at room temperature, alkaline salt solution such as sodium carbonate,

potable water and most inland bodies of fresh water. For such environments, it would not be necessary

to be concerned about sensitization. There is also generally no problem in light gauge steel since it

usually cools very quickly following welding or other exposure to high temperatures.

It is also the case that the presence of grain boundary carbides is not harmful to the high

temperature strength of stainless steels. Grades which are specifically intended for these applications

often intentionally have high carbon contents as these increases their high temperature strength and

creep resistance. These are the "H" variants such as grades 304H, 316H, 321H and 347H, and also 310.

All of these have carbon contents deliberately in the range in which precipitation will occur [7].

Because corrosion is an electrochemical process involving the flow of electric current, corrosion can be

generated by a galvanic effect which arises from the contact of dissimilar metals in an electrolyte (an

electrolyte is an electrically conductive liquid). In fact three conditions are required for galvanic

corrosion to proceed; the two metals must be widely separated on the galvanic series (see Figure 2),

they must be in electrical contact and their surfaces must be bridged by an electrically conducting fluid.

Removal of any of these three conditions will prevent galvanic corrosion [8, 9].

Figure 2: Galvanic series for metals in flowing sea water.

The obvious means of prevention is therefore to avoid mixed metal fabrications. Frequently this

is not practical, but prevention can also be by removing the electrical contact - this can be achieved by

the use of plastic or rubber washers or sleeves or by ensuring the absence of the electrolyte such as by

improvement to draining or by the use of protective hoods. This effect is also dependent upon the

relative areas of the dissimilar metals. If the area of the less noble material (the anodic material, further

towards the right in Figure 2) is large compared to that of the more noble (cathodic) the corrosive

Corrosion Analysis of Stainless Steel 513

effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in

contact with a small area of less noble will accelerate the galvanic corrosion rate. For example it is

common practice to fasten aluminum sheets with stainless steel screws, but aluminum screws in a large

area of stainless steel are likely to rapidly corrode.

This combines elements of pitting, crevice and galvanic corrosion, and occurs where small particles of

foreign matter, in particular carbon steel, are left on a stainless steel surface. The attack starts as a

galvanic cell - the particle of foreign matter is anodic and hence likely to be quickly corroded away, but

in severe cases a pit may also form in the stainless steel, and pitting corrosion can continue from this

point. The most prevalent cause is debris from nearby grinding of carbon steel, or use of tools

contaminated with carbon steel. For this reason some fabricators have dedicated stainless steel

workshops where contact with carbon steel is totally avoided. All workshops and warehouses handling

or storing stainless steels must also be aware of this potential problem, and take precautions to prevent

it. Protective plastic, wood or carpet strips can be used to prevent contact between stainless steel

products and carbon steel storage racks. Other handling equipment to be protected includes fork lift

tines and crane lifting fixtures. Clean fabric slings have often been found to be a useful alternative [10].

Passivation and Pickling

If stainless steel does become contaminated by carbon steel debris this can be removed by

passivation with dilute nitric acid or pickling with a mix of hydrofluoric and nitric acids [11].

The article analyses the corrosion of chimney parts made of stainless steel under the conditions of acid

environment. Several types of stainless steel were examined. The chemical composition of the types of

steel researched is presented in Table 1.

Table 1: Chemical composition of stainless steel

a 0.08 0.48 1.45 0.20 0.065 18.48 2.05 10.89 0.07 0.52 0.01

b 0.06 0.54 1.92 0.26 0.41 17.85 2.19 10.95 0.07 0.25 0.01

c 0.06 0.38 1.59 0.19 0.007 18.95 0.32 9.05 0.07 0.01 0.00

The resistance of steels for corrosion was examined using 10 % sulfuric acid with impurities of

sodium chloride. The samples were preserved in the solution for a particular period and the loss of the

weight of a sample was indicated. The software “Statistics” was used for carrying out statistic

calculations.

The results of the analysis are indicated in Fig. 1. The findings clearly indicate that stainless

steel resistance to corrosion in 10% sulfuric acid depends on its chemical composition. The most

resistant steels are the ones, containing titanium and molybdenum. The total loss of the weight of the

sample, including 0, 52 % titanium and 2, 05 % molybdenum, kept in the sulfuric acid for 30 days and

night was 1, 97 mg/dm2. The total loss of weight of stainless steel sample containing 0, 01% of

titanium and 0, 32% of molybdenum, examined under the same conditions was135 mg/dm2. The

impurity of chlorides of the solution decreases the resistance of stainless steel to corrosion.

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Figure 1: The loss of chromium, nickel and iron within the samples of stainless steel, affected by 10 %

sulfuric acid with impurities of sodium chloride. a, b, c indicate a steel sample number.

The existence of titanium and molybdenum in the composition of stainless steel increases its

resistance to corrosion noticeably if the amount of chloride impurities does not exceed 2 %. The most

resistant steels have mono-phase structure; however, under the conditions of higher concentration of

chloride impurities, the weight loss of all the composition samples is similar. The addition of

molybdenum decreases the resistance to corrosion of the steels of 316, when they are affected by nitric

acid of higher concentration, but the amount of nitrogen compounds is small.

The structure of the type of stainless steel, containing a bigger amount of molybdenum and

titanium in its composition (sample 1) is smaller in comparison to sample 3 (Fig. 2) that contains a

smaller concentration of titanium and molybdenum. Usually the types of steel that have bigger iron

granules are less corrosion resistant [12].

The chemical composition of sample 1 corresponds to the type of steel 1.4301. The pictures of

parts of the insert for a chimney made of this type of steel, used for 6 years are displayed in Fig. 3.

Figure 2: The structure of stainless steel samples

Figure 3: The pictures of the chimney insert after 6 years of usage

The picture reveals that the lowest part of the chimney near the cleaning opening was most

corrosion affected. Here the flowing condensate does not dry up for a long time as hot products of

combustion do not pass. The Fig. 3 presents affected spot-welding places. Steels containing 17 % of

chromium are assigned to a poli-ferrite or ferrite class. If the amount of carbon is close to 0, 1 %, steel

can be assigned to a class of semi-ferrites. This type of steel is marked by a reduced resistance to

corrosion in the places of welding seams. This is partially determined by metal granules, increased due

to high temperature during the process of welding [3]. The intercrystalline corrosion is more easily

observed in these types of steel. The addition of titanium increases corrosion resistance, but the

minimum margin of titanium should be five times bigger in comparison to carbon. Fig. 3 presents a

part made of this composition of steel, containing 0,08 % of carbon, 0,39 % of titanium, 18,63 % of

chromium, 9,12 % of nickel, 0,30 % of molybdenum, 1,05 % of manganese, 0,35 % of silicon, 0,25 %

of copper, 0,116 % of aluminum, 0,09 % of vanadium, 0,03 % of niobium, 0.07 % of cobalt and 0,50

% of wolfram. The article presents the composition of steels indicates applying the method of a

spectral analysis with a spectrum analyzer Belec 2000.

• The most stable steels in the aggressive environment of condensate are those with a bigger

amount of titanium and molybdenum.

• Chloride ions, existing in combustion, decrease corrosion resistance of chromic-nickel steels.

• It is necessary to find out for solutions how to increase corrosion resistance of welded parts.

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