**2.1.2 Martensitic stainless steels**

Martensitic stainless steels are similar in structure to the ferritics, but by the addition of more carbon, they can be hardened and strengthened by heat treatment in the same way to carbon steels. They are classified as a "hard" ferro-magnetic group. Their corrosion resistance is inferior to that of austenitic stainless steels, therefore they are generally used in mildly corrosive environments. The martensitic grades commonly used are AISI 403, AISI 410 and AISI 420 and they are widely used for cutting and grinding applications, especially for knives.

### **2.1.3 Austenitic stainless steels**

Austenitic stainless steel is non-magnetic (*i.e.*, has a low "permeability") and has excellent ductility, formability and toughness, even at cryogenic temperatures. Depending on the nickel content the austenitics respond to cold working by increases in strength, which can surprisingly be useful in severe forming operations, avoiding premature tearing and cracking. The most representative austenitic grades are AISI 304 and AISI 316. Most stainless steel containers, pipeworks and food contact equipments are manufactured from the most representative austenitic grades either 304 or 316 type austenitic stainless steels. They are widely used in food processing, beverage industry (Mai et al., 2006) and others: bulk storage and transportation and many other applications. For example the sugar, starch and wine industry requires equipments with good corrosion resistance and thus have adopted the AISI 304 and AISI 316 stainless steel. Beer is produce using raw materials like water, barley, hops, malt by fermentation, filtration, canning and sterilization process. Beer, wort and mashed grain is generally not corrosive to stainless steel such as 304, even though the process vessels and pipe systems during brewing operate from low temperatures up to the boiling point. In sections with temperatures above 60°C, there is a risk of chloride-induced stress corrosion cracking, often from the outside, in case the insulation material gets wet.

### **2.1.4 Duplex stainless steels**

The duplex stainless steels have a balanced or mixed structure of austenite and ferrite and as a result have characteristics of both "basic" types. Just like the ferritics, they are ferromagnetic with a good formability and weldability as the austenitics. In adition, the duplex stainless steels have the general corrosion resistance similar to or better than that of AISI 304 and 316 (Tavares et al., 2010). Examples of duplex grades are AISI 2304, AISI 2205, AISI 2507.

Electrochemical Behaviour of AISI 304 Stainless Steel

**2.3.1 Electrochemical corrosion** 

O2+ H2O + 4e-

et al., 2008).

**2.3.2 Corrosion severity** 

process on the surface (Yuan & Pehkonen, 2007).

**3. The fungi relevance on food industry** 

Immersed in Mixtures Consisting by Biocide and Fungal Suspensions 101

In food industry due to specific requirements (moisture, aqueous solutions, significant concentrations of organic/inorganic acids, sulfiting agents used to treat foods, high in salinity content, highly concentrated disinfecting solutions, *etc.*) the stainless steel surface is corroded through exposure in different environments. Most corrosion processes are electrochemical (corrosion of metals involves electrons transfer) (see equations: 1 - 4).

Me → Men+ + ne- (anodic reaction; metal dissolution) (1)

 *e.g.* Fe →Fe+2+ 2e- (2)

The excess of electrons resulting from the reaction loads the metal negatively and the anodic process can not continue within measurable intensity, wheareas on the metallic surface there is a cathodic process of depolarizing agent reduction (*e.g.* hydrogen ions). The reduction of hydrogen ions is the most common reaction that accompanies aqueous corrosion of metals. During corrosion processes the hydrogen ions can be absorbed on the metallic surface and diffuses inside, while the rest of hydrogen forms gas molecules and escapes. The hydrogen dissolved in metals significantly affects their mechanical properties, composition and structure of passive films formed on the surface. The electrochemical corrosion initiation at sub-microscopic level involves several phenomena starting from breakdown of passive film in a stochastic and sporadic way to localized dissolution of oxide covered metal and mass transport of atoms across the surface to support the continuing dissolution process (Marcus

The surface corrosion is an issue in many industries and it is even a greater challenge on the food processing industry, where it can cause direct (equipments failure) or indirect damage through the loss of production time for maintenance of equipments, the risk of food products contamination by corrosion products or endanger workers*'* safety and operational security (Holah & Thorpe, 1990; Ofoegbu et al., 2011). When the equipments begin the long walk down the dark road of corrosion, small amounts of metallic elements in the alloy may migrate into foodstuffs from equipment leading to human ingestion and can cause adverse health effects. The severity of the corrosion can be estimated easier through corrosion rate (*Vcorr*). A corrosion rate for food equipments surfaces which exceeds 0.02 mm/y reduces the equipments lifetime (Fontana, 1987). The corrosion on metallic surfaces can lead to the formation and expansion of cavities and grooves. This in turn provides breeding sites for microorganisms, thereby compromising the efficacy of cleaning and disinfection procedures and encourages more biofilm adhesion and biofilm resistance to detachment. The biofilms probably do not participate directly in the corrosion process, but they can lead to some changes of the interfacial environment by the increase of cells concentration that facilitates the corrosion

The living world is classified in three domains such as the *Bacteria*, the *Archaea* and the *Eukaryota* (Woese, 1990). Some authors argued that these species represent separate lines of

→4 OH- (catodic reaction; consume the electrons) (3)

95

Me + nH2O → Me(OH)n + nH+ + ne- (overall reaction) (4)

#### **2.2 Factors that influence the hygiene of food contact surfaces**

Food contact surfaces include any item that comes in direct contact or could potentially be in contact with the exposed food. The surfaces must be cleaned and disinfected before beginning each day's work, after each use and before changing to a different type of food, such as raw meats to vegetables. The cleaning and disinfection in food industries is an issue of utmost importance since high hygienic standards assure the safety and quality of end products and therefore the consumer's health.` This subchapter will shortly describe the passive film, the finishing, roughness and cleanability on the surfaces.

### **2.2.1 Passive film**

The stainless steel corrosion resistance arises from a protective layer formed on metallic surfaces. The protective layer, sometimes only a few nm in thickness is called a passive film and it is formed instantaneously in an oxidizing atmosphere such as air, water or other fluids that contain oxygen (Olsson et Landolt, 2003). Once the layer is formed, the metallic surface becomes *passivated* and the oxidation or *rusting* rate will slow down to less than 0.05 mm/year. The passive film has normally considerable practical importance also in food production quality since the thin film protects the underlying metal from corrosion. The protective film is strongly adherent on the metallic surfaces and presents longer stability in time having some impact on food safety.

#### **2.2.2 Surfaces finishing and cleanability**

The surfaces finishing grade renders the quality of the surfaces described by roughness. The surfaces roughness is characterized in two main directions: one perpendicular to the surface, described as height deviation and the second one in the plane of the surface, described by spatial parameters and identified as texture (Round et al., 2001). The surfaces status was defined by the following characteristic parameters: *Ra* known as the arithmetic average height parameter, *Rq* known as the root mean square and *Rmax* is the difference in height between the highest and lowest points on the surface relative to the mean plane. The *Ra* is the most universally roughness parameter used (Buchalla et al., 2000; Turssi et al., 2001) to with a general quality surface control (Whitehead & Verran, 2006). The variation of the average roughness can be useful to express the corrosion process on the surfaces (Sánchez et al., 2008). At the same time the roughness is important for surfaces cleanability (Leclerq-Perlat & Lalande,1994; Boulange-Petermann et al., 2004). A lower roughness allows for easy cleaning intended to eliminate bacteria, which considerably reduces service cost of materials. Surfaces in contact with the food product should have an standard roughness (*Ra*) value. For surfaces coming in contact with a product having a large area, the *Ra* should be less than 0.8 μm (Holah and Thorpe, 1990). Under special conditions a roughness higher than 0.8 μm can be accepted in case test results prove that the required cleaning capacity is reached.

#### **2.3 The metallic surfaces corrosion**

The corrosion is the destruction or the deterioration of metallic materials by direct chemical, electrochemical or biochemical reactions with different environments (Landoulsi et al., 2008; Osarolube et al., 2008). The corrosion processes are a very broad and complex phenomenon which can be uniform (*e.g.* general corrosion), located (*e.g*. crevice and pitting corrosion) and or in any other way.

#### **2.3.1 Electrochemical corrosion**

100 Food Industrial Processes – Methods and Equipment

Food contact surfaces include any item that comes in direct contact or could potentially be in contact with the exposed food. The surfaces must be cleaned and disinfected before beginning each day's work, after each use and before changing to a different type of food, such as raw meats to vegetables. The cleaning and disinfection in food industries is an issue of utmost importance since high hygienic standards assure the safety and quality of end products and therefore the consumer's health.` This subchapter will shortly describe the

The stainless steel corrosion resistance arises from a protective layer formed on metallic surfaces. The protective layer, sometimes only a few nm in thickness is called a passive film and it is formed instantaneously in an oxidizing atmosphere such as air, water or other fluids that contain oxygen (Olsson et Landolt, 2003). Once the layer is formed, the metallic surface becomes *passivated* and the oxidation or *rusting* rate will slow down to less than 0.05 mm/year. The passive film has normally considerable practical importance also in food production quality since the thin film protects the underlying metal from corrosion. The protective film is strongly adherent on the metallic surfaces and presents longer stability in

The surfaces finishing grade renders the quality of the surfaces described by roughness. The surfaces roughness is characterized in two main directions: one perpendicular to the surface, described as height deviation and the second one in the plane of the surface, described by spatial parameters and identified as texture (Round et al., 2001). The surfaces status was defined by the following characteristic parameters: *Ra* known as the arithmetic average height parameter, *Rq* known as the root mean square and *Rmax* is the difference in height between the highest and lowest points on the surface relative to the mean plane. The *Ra* is the most universally roughness parameter used (Buchalla et al., 2000; Turssi et al., 2001) to with a general quality surface control (Whitehead & Verran, 2006). The variation of the average roughness can be useful to express the corrosion process on the surfaces (Sánchez et al., 2008). At the same time the roughness is important for surfaces cleanability (Leclerq-Perlat & Lalande,1994; Boulange-Petermann et al., 2004). A lower roughness allows for easy cleaning intended to eliminate bacteria, which considerably reduces service cost of materials. Surfaces in contact with the food product should have an standard roughness (*Ra*) value. For surfaces coming in contact with a product having a large area, the *Ra* should be less than 0.8 μm (Holah and Thorpe, 1990). Under special conditions a roughness higher than 0.8 μm can be accepted in case test results prove that the required cleaning capacity is

The corrosion is the destruction or the deterioration of metallic materials by direct chemical, electrochemical or biochemical reactions with different environments (Landoulsi et al., 2008; Osarolube et al., 2008). The corrosion processes are a very broad and complex phenomenon which can be uniform (*e.g.* general corrosion), located (*e.g*. crevice and pitting corrosion) and

**2.2 Factors that influence the hygiene of food contact surfaces** 

passive film, the finishing, roughness and cleanability on the surfaces.

**2.2.1 Passive film** 

94

reached.

or in any other way.

time having some impact on food safety.

**2.2.2 Surfaces finishing and cleanability** 

**2.3 The metallic surfaces corrosion** 

In food industry due to specific requirements (moisture, aqueous solutions, significant concentrations of organic/inorganic acids, sulfiting agents used to treat foods, high in salinity content, highly concentrated disinfecting solutions, *etc.*) the stainless steel surface is corroded through exposure in different environments. Most corrosion processes are electrochemical (corrosion of metals involves electrons transfer) (see equations: 1 - 4).

$$\text{Me} \rightarrow \text{Me}^{\bullet \ast} + \text{ne} \text{ (anodic reaction; metal dissociation)}\tag{1}$$

$$e.g.\,\text{Fe} \rightarrow \text{Fe}\*\text{2} + \text{2e}\*\tag{2}$$

$$\text{O}\_2 + \text{H}\_2\text{O} + 4\text{e} \rightarrow 4\text{OH} \cdot \text{(catoodic reaction; course the electrons)}\tag{3}$$

$$\text{Me} \vdash \text{nH} \text{xO} \rightarrow \text{Me(OH)} \text{n} + \text{nH}^\* + \text{ne} \quad \text{(overall reaction)} \tag{4}$$

The excess of electrons resulting from the reaction loads the metal negatively and the anodic process can not continue within measurable intensity, wheareas on the metallic surface there is a cathodic process of depolarizing agent reduction (*e.g.* hydrogen ions). The reduction of hydrogen ions is the most common reaction that accompanies aqueous corrosion of metals. During corrosion processes the hydrogen ions can be absorbed on the metallic surface and diffuses inside, while the rest of hydrogen forms gas molecules and escapes. The hydrogen dissolved in metals significantly affects their mechanical properties, composition and structure of passive films formed on the surface. The electrochemical corrosion initiation at sub-microscopic level involves several phenomena starting from breakdown of passive film in a stochastic and sporadic way to localized dissolution of oxide covered metal and mass transport of atoms across the surface to support the continuing dissolution process (Marcus et al., 2008).

#### **2.3.2 Corrosion severity**

The surface corrosion is an issue in many industries and it is even a greater challenge on the food processing industry, where it can cause direct (equipments failure) or indirect damage through the loss of production time for maintenance of equipments, the risk of food products contamination by corrosion products or endanger workers*'* safety and operational security (Holah & Thorpe, 1990; Ofoegbu et al., 2011). When the equipments begin the long walk down the dark road of corrosion, small amounts of metallic elements in the alloy may migrate into foodstuffs from equipment leading to human ingestion and can cause adverse health effects. The severity of the corrosion can be estimated easier through corrosion rate (*Vcorr*). A corrosion rate for food equipments surfaces which exceeds 0.02 mm/y reduces the equipments lifetime (Fontana, 1987). The corrosion on metallic surfaces can lead to the formation and expansion of cavities and grooves. This in turn provides breeding sites for microorganisms, thereby compromising the efficacy of cleaning and disinfection procedures and encourages more biofilm adhesion and biofilm resistance to detachment. The biofilms probably do not participate directly in the corrosion process, but they can lead to some changes of the interfacial environment by the increase of cells concentration that facilitates the corrosion process on the surface (Yuan & Pehkonen, 2007).
